diff --git a/.editorconfig b/.editorconfig
index 0da8f80..f8ae8b4 100644
--- a/.editorconfig
+++ b/.editorconfig
@@ -1,3 +1,5 @@
 [*]
 indent_size = 2
 indent_style = space
+trim_trailing_whitespace = true
+insert_final_newline = true
diff --git a/.github/workflows/test.yml b/.github/workflows/test.yml
new file mode 100644
index 0000000..dc15109
--- /dev/null
+++ b/.github/workflows/test.yml
@@ -0,0 +1,31 @@
+name: test
+
+on:
+  workflow_dispatch:
+  schedule:
+    - cron: '3 0 2 * *'
+  push:
+
+jobs:
+  test:
+    runs-on: ubuntu-latest
+    name: Ruby ${{ matrix.ruby }}
+    strategy:
+      matrix:
+        ruby:
+          - '2.7'
+          - '3.0'
+          - '3.1'
+          - '3.2'
+          - '3.3'
+    steps:
+      - uses: actions/checkout@v4
+      - name: Set up RUby
+        uses: ruby/setup-ruby@v1
+        with:
+          ruby-version: ${{ matrix.ruby }}
+          bundler-cache: true
+      - name: lint
+        run: bundle exec rake standard
+      - name: test
+        run: bundle exec rake spec
diff --git a/.rubocop.yml b/.rubocop.yml
new file mode 100644
index 0000000..1ea9b91
--- /dev/null
+++ b/.rubocop.yml
@@ -0,0 +1,5 @@
+require:
+  - standard
+
+inherit_gem:
+  standard: config/base.yml
diff --git a/.standard.yml b/.standard.yml
new file mode 100644
index 0000000..3741b18
--- /dev/null
+++ b/.standard.yml
@@ -0,0 +1,3 @@
+# For available configuration options, see:
+#   https://github.com/testdouble/standard
+ruby_version: 2.7
diff --git a/.travis.yml b/.travis.yml
deleted file mode 100644
index b1aa6b8..0000000
--- a/.travis.yml
+++ /dev/null
@@ -1,7 +0,0 @@
----
-sudo: false
-language: ruby
-cache: bundler
-rvm:
-  - 2.6.5
-before_install: gem install bundler -v 2.0.2
diff --git a/CHANGELOG.md b/CHANGELOG.md
new file mode 100644
index 0000000..49515ed
--- /dev/null
+++ b/CHANGELOG.md
@@ -0,0 +1,7 @@
+## [0.3.5] 2024-08-16
+
+- standardrb compliant
+
+## [0.1.0] 2021-04-16
+
+- initial release
diff --git a/Gemfile b/Gemfile
index 336cd1e..b05d6f2 100644
--- a/Gemfile
+++ b/Gemfile
@@ -2,3 +2,16 @@ source "https://rubygems.org"
 
 # Specify your gem's dependencies in graph.gemspec
 gemspec
+
+gem "bundler", "~> 2.0"
+gem "rake", "~> 13.0"
+
+gem "standard", "~> 1.37"
+gem "rubocop", "~> 1.62"
+gem "yard"
+
+group :test do
+  gem "minitest", "~> 5.0"
+  gem "minitest-reporters", "~> 1"
+  gem "minitest-power_assert"
+end
diff --git a/Rakefile b/Rakefile
index f7ff562..4f377b3 100644
--- a/Rakefile
+++ b/Rakefile
@@ -8,9 +8,11 @@ Rake::TestTask.new(:spec) do |t|
   t.test_files = FileList["spec/**/*_spec.rb"]
 end
 
+require "standard/rake"
+
 YARD::Rake::YardocTask.new do |t|
   t.files = ["lib/**/*.rb"]
   t.options = ["--any", "--extra", "--opts"]
 end
 
-task :default => :spec
+task default: :spec
diff --git a/exe/simple-hd-graph b/exe/simple-hd-graph
index 6e98ee2..4fd01ec 100755
--- a/exe/simple-hd-graph
+++ b/exe/simple-hd-graph
@@ -1,8 +1,8 @@
 #! /usr/bin/env ruby
 
-$LOAD_PATH.unshift File.absolute_path(File.join(__dir__, '../lib'))
+$LOAD_PATH.unshift File.absolute_path(File.join(__dir__, "../lib"))
 
-require 'simple-hd-graph'
+require "simple-hd-graph"
 
 Version = SimpleHdGraph::VERSION
 cmd = SimpleHdGraph::Command.new
diff --git a/lib/simple-hd-graph.rb b/lib/simple-hd-graph.rb
index 462df02..58f1506 100644
--- a/lib/simple-hd-graph.rb
+++ b/lib/simple-hd-graph.rb
@@ -3,6 +3,6 @@ class Error < StandardError; end
   # Your code goes here...
 end
 
-Dir.glob(__dir__ + '/simple-hd-graph/**/*.rb').each { |f|
-  require f.sub(/\.rb\z/, '')
+Dir.glob(__dir__ + "/simple-hd-graph/**/*.rb").each { |f|
+  require f.sub(/\.rb\z/, "")
 }
diff --git a/lib/simple-hd-graph/command.rb b/lib/simple-hd-graph/command.rb
index a2912e0..59f3c3b 100644
--- a/lib/simple-hd-graph/command.rb
+++ b/lib/simple-hd-graph/command.rb
@@ -1,9 +1,8 @@
-require 'optparse'
+require "optparse"
 
 module SimpleHdGraph
-  class Error < StandardError; end
   class FileNotExist < Error; end
-  class DirectoryNotExist < Error; end
+  class DirectoryNotExist < Error; end # rubocop:disable Layout/EmptyLineBetweenDefs
 
   class Command
     #
@@ -39,7 +38,7 @@ def parse(argv)
     # @return [String]
     #
     def stream
-      if (@dir)
+      if @dir
         reader.read_dir(@dir)
       else
         reader.read_file(@file)
@@ -58,25 +57,25 @@ def start
     # :reek:NestedIterators, :reek:DuplicateMethodCall
     def opts
       OptionParser.new do |opt|
-        opt.on('-d DIR', '--dir', 'dirname') { |value|
+        opt.on("-d DIR", "--dir", "dirname") { |value|
           if File.exist?(value) && File.directory?(value)
             @dir = value
           else
             raise DirectoryNotExist, value
           end
         }
-        opt.on('-f FILE', '--file', 'filename') { |value|
+        opt.on("-f FILE", "--file", "filename") { |value|
           if File.exist?(value) && File.file?(value)
             @file = value
           else
             raise FileNotExist, value
           end
         }
-        opt.on('-r RENDERER', '--renderer', 'renderer') { |value|
+        opt.on("-r RENDERER", "--renderer", "renderer") { |value|
           begin
             @renderer = SimpleHdGraph::Renderer.method(value)
           rescue NameError
-            STDERR.puts "[Warining] renderer `#{value}` not found. falling back to :plantuml"
+            warn "[Warining] renderer `#{value}` not found. falling back to :plantuml"
           end
         }
       end
diff --git a/lib/simple-hd-graph/context.rb b/lib/simple-hd-graph/context.rb
index 8b24082..2f0b9ce 100644
--- a/lib/simple-hd-graph/context.rb
+++ b/lib/simple-hd-graph/context.rb
@@ -1,4 +1,4 @@
-require 'simple-hd-graph/node'
+require "simple-hd-graph/node"
 
 module SimpleHdGraph
   class ContextNode < Node
@@ -39,15 +39,14 @@ def set_depends(depends)
       @depends = depends
     end
 
-    # :reek:NestedIterators, :reek:TooManyStatements
     def refill_relation
-      @resource  ||= []
+      @resource ||= []
       @relations ||= []
       @resources.each { |resource|
         dependencies = resource.has
-        if dependencies
+        if dependencies.respond_to? :each
           dependencies.each { |dependency|
-            @relations << { resource.id => @resource_dict[dependency] }
+            @relations << {resource.id => @resource_dict[dependency]}
           }
         end
       }
diff --git a/lib/simple-hd-graph/node.rb b/lib/simple-hd-graph/node.rb
index 603eed8..9846d05 100644
--- a/lib/simple-hd-graph/node.rb
+++ b/lib/simple-hd-graph/node.rb
@@ -1,8 +1,8 @@
-require 'dry/inflector'
+require "dry/inflector"
 
 module SimpleHdGraph
   class Node
-    class RequiredFieldNotFilled < StandardError; end
+    class RequiredFieldNotFilled < Error; end
 
     class << self
       #
@@ -13,7 +13,7 @@ def required(*names)
       end
     end
 
-    CAMELIZE_SEPARATOR = ' ,.、。'
+    CAMELIZE_SEPARATOR = /[ ,.、。]/
 
     def initialize
       @inflector = Dry::Inflector.new
@@ -22,27 +22,26 @@ def initialize
     #
     # @param struct [Hash]
     #
-    # :reek:TooManyStatements
     def load(struct)
       klass = self.class
 
-      required_fields = if klass.instance_variables.grep(/@required_fields/).size > 0
-                          klass.instance_variable_get('@required_fields')
-                        else
-                          nil
-                        end
-
-      if required_fields.is_a? Array
-        filled = required_fields.all? {|field|
-          if struct.has_key? field
-            true
-          else
-            raise RequiredFieldNotFilled, field
-          end
-        }
-      else
-        filled = true
-      end
+      required_fields =
+        if klass.instance_variables.grep(/@required_fields/).size > 0
+          klass.instance_variable_get(:@required_fields)
+        end
+
+      filled =
+        if required_fields.is_a? Array
+          required_fields.all? { |field|
+            if struct.has_key? field
+              true
+            else
+              raise RequiredFieldNotFilled, field
+            end
+          }
+        else
+          true
+        end
 
       @content = struct if filled
     end
@@ -52,7 +51,7 @@ def load(struct)
     # @return [String]
     #
     def camelize(str)
-      @inflector.camelize(@inflector.underscore(str.gsub(/[#{CAMELIZE_SEPARATOR}]/, '_')))
+      @inflector.camelize(@inflector.underscore(str.gsub(CAMELIZE_SEPARATOR, "_")))
     end
   end
 end
diff --git a/lib/simple-hd-graph/parser.rb b/lib/simple-hd-graph/parser.rb
index f231272..5c37161 100644
--- a/lib/simple-hd-graph/parser.rb
+++ b/lib/simple-hd-graph/parser.rb
@@ -1,34 +1,32 @@
-require 'yaml'
+require "yaml"
 
 module SimpleHdGraph
   #
   # parse for YAML string
   #
-  # :reek:InstanceVaariableAssumption
   class Parser
-    KEYWORD_ID        ||= 'id'.freeze
-    KEYWORD_RESOURCES ||= 'resources'.freeze
-    KEYWORD_DEPENDS   ||= 'depends'.freeze
+    KEYWORD_ID = "id".freeze
+    KEYWORD_RESOURCES = "resources".freeze
+    KEYWORD_DEPENDS = "depends".freeze
 
     #
     # @param document [String]
     #
-    # :reek:TooManyStatements
     def parse(document)
       contexts = []
 
       YAML.load_stream(document) do |node|
         next unless node
 
-        context   = nil
+        context = nil
         resources = nil
-        depends   = nil
+        depends = nil
 
         node.each_pair { |key, value|
           case key
           when KEYWORD_ID
             context = ContextNode.new
-            context.load({ id: value })
+            context.load({id: value})
           when KEYWORD_DEPENDS
             depends = value
           when KEYWORD_RESOURCES
@@ -62,7 +60,7 @@ def pack_depends_into_context(depends, context)
     def pack_resources_into_context(resources, context)
       resources.each { |key, resource|
         rn = ResourceNode.new
-        rn.load_with_context({ id: context.id }, { key => resource })
+        rn.load_with_context({id: context.id}, {key => resource})
         context << rn
       }
     end
@@ -71,7 +69,7 @@ def pack_resources_into_context(resources, context)
     # @param contexts [Array]
     #
     def refill_relation(contexts)
-      contexts.each {|context|
+      contexts.each { |context|
         context.refill_relation
       }
     end
@@ -83,9 +81,9 @@ def refill_depends(contexts)
       contexts.map { |context|
         if context.depends
           regularized = context.depends.map { |dependee|
-            d = contexts.select { |c|
+            d = contexts.find { |c|
               c.alias == dependee
-            }.first
+            }
 
             {
               context.id => d ? d.id : dependee
diff --git a/lib/simple-hd-graph/reader.rb b/lib/simple-hd-graph/reader.rb
index 543feda..8ba6482 100644
--- a/lib/simple-hd-graph/reader.rb
+++ b/lib/simple-hd-graph/reader.rb
@@ -4,7 +4,6 @@ class Reader
     # @param file [String]
     # @return [String]
     #
-    # :reek:UtilityFunction
     def read_file(file)
       File.read(file)
     end
@@ -14,7 +13,7 @@ def read_file(file)
     # @return [String]
     #
     def read_dir(dir)
-      Dir.glob("#{dir}/**/*.{yml,yaml}").map {|file|
+      Dir.glob("#{dir}/**/*.{yml,yaml}").map { |file|
         read_file(file)
       }.join("---\n")
     end
diff --git a/lib/simple-hd-graph/renderer/plantuml.rb b/lib/simple-hd-graph/renderer/plantuml.rb
index 4c96bcb..1725bed 100644
--- a/lib/simple-hd-graph/renderer/plantuml.rb
+++ b/lib/simple-hd-graph/renderer/plantuml.rb
@@ -1,4 +1,4 @@
-Dir.glob(File.join(__dir__, '**/*.rb')).each { |f| require f }
+Dir.glob(File.join(__dir__, "plantuml/*.rb")).sort.each { |f| require f }
 
 module SimpleHdGraph
   module Renderer
diff --git a/lib/simple-hd-graph/renderer/plantuml/context.rb b/lib/simple-hd-graph/renderer/plantuml/context.rb
index 665ae0d..40441eb 100644
--- a/lib/simple-hd-graph/renderer/plantuml/context.rb
+++ b/lib/simple-hd-graph/renderer/plantuml/context.rb
@@ -9,24 +9,33 @@ def initialize
         #
         # @param node [ContextNode]
         #
-        # :reek:FeatureEnvy, :reek:DuplicateMethodCall
         def render(node)
-          resources = node.resources.map { |resource|
-            indent_resource(resource)
-          }.join if node.resources.size > 0
-          relations = node.relations.map { |relation|
-            render_relation(relation)
-          }.join("\n") if node.relations.size > 0
-          depends = node.depends.map { |depending|
-            render_depends(depending)
-          }.join("\n") if node.depends.size > 0
-          (<<-EOD).gsub(/^$\n/, '')
-rectangle \"#{node.alias}\" as #{node.id} {
-#{resources}
-#{relations}
-}
-#{depends}
-EOD
+          resources =
+            if node.resources.size > 0
+              node.resources.map { |resource|
+                indent_resource(resource)
+              }.join
+            end
+          relations =
+            if node.relations.size > 0
+              node.relations.map { |relation|
+                render_relation(relation)
+              }.join("\n")
+            end
+          depends =
+            if node.depends.size > 0
+              node.depends.map { |depending|
+                render_depends(depending)
+              }.join("\n")
+            end
+
+          <<~EOD.gsub(/^$\n/, "")
+            rectangle "#{node.alias}" as #{node.id} {
+            #{resources}
+            #{relations}
+            }
+            #{depends}
+          EOD
         end
 
         #
diff --git a/lib/simple-hd-graph/renderer/plantuml/resource.rb b/lib/simple-hd-graph/renderer/plantuml/resource.rb
index 5b9703c..5e22717 100644
--- a/lib/simple-hd-graph/renderer/plantuml/resource.rb
+++ b/lib/simple-hd-graph/renderer/plantuml/resource.rb
@@ -10,11 +10,11 @@ def render(node)
           content = node.content.map { |key, value|
             "  #{key}: #{value}"
           }.join("\n")
-          <<EOD
-object \"#{node.alias}\" as #{node.id} {
-#{content}
-}
-EOD
+          <<~EOD
+            object "#{node.alias}" as #{node.id} {
+            #{content}
+            }
+          EOD
         end
       end
     end
diff --git a/lib/simple-hd-graph/resource.rb b/lib/simple-hd-graph/resource.rb
index a836c37..e8ef9c0 100644
--- a/lib/simple-hd-graph/resource.rb
+++ b/lib/simple-hd-graph/resource.rb
@@ -1,4 +1,4 @@
-require 'simple-hd-graph/node'
+require "simple-hd-graph/node"
 
 module SimpleHdGraph
   class ResourceNode < Node
@@ -24,7 +24,7 @@ def alias
     def id
       id = camelize(context)
       id[0] = id[0].downcase
-      [id, camelize(self.alias)].join('').freeze
+      [id, camelize(self.alias)].join("").freeze
     end
 
     #
@@ -38,8 +38,8 @@ def context
     # @return [Hash]
     #
     def content
-      @content.values.first.select { |key, value|
-        key != 'has'
+      @content.values.first.select { |key, value| # rubocop:disable Style/HashExcept
+        key != "has"
       }
     end
 
@@ -47,7 +47,7 @@ def content
     # @return [Array]
     #
     def has
-      @content.values.first['has']
+      @content.values.first["has"]
     end
   end
 end
diff --git a/lib/simple-hd-graph/version.rb b/lib/simple-hd-graph/version.rb
index bbf6695..487c9aa 100644
--- a/lib/simple-hd-graph/version.rb
+++ b/lib/simple-hd-graph/version.rb
@@ -1,3 +1,3 @@
 module SimpleHdGraph
-  VERSION = "0.3.4"
+  VERSION = "0.3.5"
 end
diff --git a/simple-hd-graph.gemspec b/simple-hd-graph.gemspec
index c2d6607..0ed70ed 100644
--- a/simple-hd-graph.gemspec
+++ b/simple-hd-graph.gemspec
@@ -3,36 +3,29 @@ $LOAD_PATH.unshift(lib) unless $LOAD_PATH.include?(lib)
 require "simple-hd-graph/version"
 
 Gem::Specification.new do |spec|
-  spec.name          = "simple-hd-graph"
-  spec.version       = SimpleHdGraph::VERSION
-  spec.authors       = ["wtnabe"]
-  spec.email         = ["wtnabe@gmail.com"]
+  spec.name = "simple-hd-graph"
+  spec.version = SimpleHdGraph::VERSION
+  spec.authors = ["wtnabe"]
+  spec.email = ["18510+wtnabe@users.noreply.github.com"]
 
-  spec.summary       = %q{single-tier hierary and simplex direction graph DSL}
-  spec.description   = %q{parse single-tier hierarchy and simplex direction graph from YAML, and generate PlantUML with rectangle and object}
-  spec.homepage      = "https://github.com/wtnabe/simple-hd-graph"
+  spec.summary = "single-tier hierary and simplex direction graph DSL"
+  spec.description = "parse single-tier hierarchy and simplex direction graph from YAML, and generate PlantUML with rectangle and object"
+  spec.homepage = "https://github.com/wtnabe/simple-hd-graph"
 
   spec.metadata["allowed_push_host"] = "https://rubygems.org"
 
   spec.metadata["homepage_uri"] = spec.homepage
   spec.metadata["source_code_uri"] = spec.homepage
-  spec.metadata["changelog_uri"] = spec.metadata["source_code_uri"] + "/blob/master/CHANGELOG.md"
+  spec.metadata["changelog_uri"] = spec.metadata["source_code_uri"] + "/blob/main/CHANGELOG.md"
 
   # Specify which files should be added to the gem when it is released.
   # The `git ls-files -z` loads the files in the RubyGem that have been added into git.
-  spec.files         = Dir.chdir(File.expand_path('..', __FILE__)) do
+  spec.files = Dir.chdir(File.expand_path("..", __FILE__)) do
     `git ls-files -z`.split("\x0").reject { |f| f.match(%r{^(test|spec|features)/}) }
   end
-  spec.bindir        = "exe"
-  spec.executables   = spec.files.grep(%r{^exe/}) { |f| File.basename(f) }
+  spec.bindir = "exe"
+  spec.executables = spec.files.grep(%r{^exe/}) { |f| File.basename(f) }
   spec.require_paths = ["lib"]
 
   spec.add_runtime_dependency "dry-inflector", "~> 0"
-
-  spec.add_development_dependency "bundler", "~> 2.0"
-  spec.add_development_dependency "rake", "~> 13.0"
-  spec.add_development_dependency "minitest", "~> 5.0"
-  spec.add_development_dependency "minitest-reporters", "~> 1"
-  spec.add_development_dependency "minitest-power_assert"
-  spec.add_development_dependency "yard"
 end
diff --git a/spec/command_spec.rb b/spec/command_spec.rb
index 4559fd2..b0e0a64 100644
--- a/spec/command_spec.rb
+++ b/spec/command_spec.rb
@@ -1,42 +1,42 @@
-require 'spec_helper'
+require "spec_helper"
 
 describe SimpleHdGraph::Command do
   def command
     @command ||= SimpleHdGraph::Command.new
   end
 
-  describe '#parse' do
-    describe 'file' do
-      describe 'exists' do
+  describe "#parse" do
+    describe "file" do
+      describe "exists" do
         it {
           assert {
-            command.parse(['-f', File.join(__dir__, 'support/example_simple.yaml')])
+            command.parse(["-f", File.join(__dir__, "support/example_simple.yaml")])
           }
         }
       end
 
-      describe 'not exist' do
+      describe "not exist" do
         it {
           assert_raises SimpleHdGraph::FileNotExist do
-            command.parse(['-f', 'notexist'])
+            command.parse(["-f", "notexist"])
           end
         }
       end
     end
 
-    describe 'dir' do
-      describe 'exists' do
+    describe "dir" do
+      describe "exists" do
         it {
           assert {
-            command.parse(['-d', File.join(__dir__, 'support')])
+            command.parse(["-d", File.join(__dir__, "support")])
           }
         }
       end
 
-      describe 'not exist' do
+      describe "not exist" do
         it {
           assert_raises SimpleHdGraph::DirectoryNotExist do
-            command.parse(['-d', 'notexist'])
+            command.parse(["-d", "notexist"])
           end
         }
       end
diff --git a/spec/context_spec.rb b/spec/context_spec.rb
index 40be13a..f3073a8 100644
--- a/spec/context_spec.rb
+++ b/spec/context_spec.rb
@@ -1,25 +1,25 @@
-require 'spec_helper'
+require "spec_helper"
 
 describe SimpleHdGraph::ContextNode do
   before {
     @node = SimpleHdGraph::ContextNode.new
   }
 
-  describe '#alias' do
-    describe 'small letter only' do
+  describe "#alias" do
+    describe "small letter only" do
       it {
-        @node.load({ id: 'foo' })
+        @node.load({id: "foo"})
         assert {
-          @node.id == 'foo'
+          @node.id == "foo"
         }
       }
     end
 
-    describe 'has white space' do
+    describe "has white space" do
       it {
-        @node.load({ id: 'foo bar' })
+        @node.load({id: "foo bar"})
         assert {
-          @node.id == 'fooBar'
+          @node.id == "fooBar"
         }
       }
     end
diff --git a/spec/node_spec.rb b/spec/node_spec.rb
index 40a2fee..6440d94 100644
--- a/spec/node_spec.rb
+++ b/spec/node_spec.rb
@@ -1,6 +1,6 @@
-require 'spec_helper'
+require "spec_helper"
 
-require 'yaml'
+require "yaml"
 
 class TestingSingleRequiredFieldNode < SimpleHdGraph::Node
   required :name
@@ -11,21 +11,21 @@ class TestingMultipleRequiredFieldsNode < SimpleHdGraph::Node
 end
 
 def testing_single_required_valid_structure
-  YAML.load(<<EOD, symbolize_names: true)
-name: foo
-EOD
+  YAML.safe_load(<<~EOD, symbolize_names: true)
+    name: foo
+  EOD
 end
 
 def testing_multiple_required_valid_structure
-  YAML.load(<<EOD, symbolize_names: true)
-id: foo
-name: bar
-EOD
+  YAML.safe_load(<<~EOD, symbolize_names: true)
+    id: foo
+    name: bar
+  EOD
 end
 
 describe SimpleHdGraph::Node do
-  describe '.required and #load' do
-    describe 'no required fields' do
+  describe ".required and #load" do
+    describe "no required fields" do
       it {
         assert {
           SimpleHdGraph::Node.new.load({})
@@ -33,19 +33,19 @@ def testing_multiple_required_valid_structure
       }
     end
 
-    describe 'single field' do
+    describe "single field" do
       before {
         @node = TestingSingleRequiredFieldNode.new
       }
 
-      describe 'filled struct' do
-        it 'valid' do
+      describe "filled struct" do
+        it "valid" do
           assert { @node.load(testing_single_required_valid_structure) }
         end
       end
 
-      describe 'empty' do
-        it 'invalid' do
+      describe "empty" do
+        it "invalid" do
           assert_raises @node.class::RequiredFieldNotFilled do
             @node.load({})
           end
@@ -53,26 +53,26 @@ def testing_multiple_required_valid_structure
       end
     end
 
-    describe 'multiple field' do
+    describe "multiple field" do
       before {
         @node = TestingMultipleRequiredFieldsNode.new
       }
 
-      describe 'filled struct' do
-        it 'valid' do
+      describe "filled struct" do
+        it "valid" do
           assert {
             @node.load(testing_multiple_required_valid_structure)
           }
         end
       end
 
-      describe 'empty' do
-        it 'invalid' do
+      describe "empty" do
+        it "invalid" do
           assert {
             begin
               @node.load({})
             rescue SimpleHdGraph::Node::RequiredFieldNotFilled => e
-              e.message == 'id'
+              e.message == "id"
             end
           }
         end
diff --git a/spec/parser_spec.rb b/spec/parser_spec.rb
index 20590f5..c7ef5c6 100644
--- a/spec/parser_spec.rb
+++ b/spec/parser_spec.rb
@@ -1,57 +1,57 @@
-require 'spec_helper'
+require "spec_helper"
 
 describe SimpleHdGraph::Parser do
   include ExampleLoader
 
   let(:parser) { SimpleHdGraph::Parser.new }
 
-  describe '#parse' do
-    describe 'simple' do
+  describe "#parse" do
+    describe "simple" do
       before {
         @nodes = parser.parse(read_example(:simple))
       }
 
-      it 'context' do
+      it "context" do
         assert {
           @nodes.all? { |context|
-            context.class == SimpleHdGraph::ContextNode
+            context.is_a?(SimpleHdGraph::ContextNode)
           }
         }
       end
 
-      it 'context id' do
+      it "context id" do
         assert {
-          @nodes.first.id == 'simple'
+          @nodes.first.id == "simple"
         }
       end
 
-      it 'resource' do
+      it "resource" do
         assert {
           @nodes.all? { |context|
             context.resources.all? { |resource|
-              resource.class == SimpleHdGraph::ResourceNode
+              resource.is_a?(SimpleHdGraph::ResourceNode)
             }
           }
         }
       end
     end
 
-    describe 'complex' do
+    describe "complex" do
       before {
         @nodes = parser.parse(read_example(:complex))
       }
 
-      it 'relations' do
+      it "relations" do
         assert {
           @nodes.first.relations == [
-            { 'example1Web' => 'example1Admin' },
-            { 'example1Web' => 'example1Storage' }
+            {"example1Web" => "example1Admin"},
+            {"example1Web" => "example1Storage"}
           ]
         }
       end
     end
 
-    describe 'depends' do
+    describe "depends" do
       before {
         @nodes = parser.parse(
           [
@@ -63,7 +63,7 @@
 
       it {
         assert {
-          @nodes.map { |node| node.depends } == [[], [{'example2' => 'example1'}]]
+          @nodes.map { |node| node.depends } == [[], [{"example2" => "example1"}]]
         }
       }
     end
diff --git a/spec/reader_spec.rb b/spec/reader_spec.rb
index fabee6b..75249fe 100644
--- a/spec/reader_spec.rb
+++ b/spec/reader_spec.rb
@@ -1,11 +1,11 @@
-require 'spec_helper'
+require "spec_helper"
 
 describe SimpleHdGraph::Reader do
-  describe '#read_dir' do
+  describe "#read_dir" do
     it {
       reader = SimpleHdGraph::Reader.new
       assert {
-        reader.read_dir(File.join(__dir__, 'support'))
+        reader.read_dir(File.join(__dir__, "support"))
       }
     }
   end
diff --git a/spec/renderer/plantuml/context_spec.rb b/spec/renderer/plantuml/context_spec.rb
index 7bf1688..f0d800d 100644
--- a/spec/renderer/plantuml/context_spec.rb
+++ b/spec/renderer/plantuml/context_spec.rb
@@ -1,4 +1,4 @@
-require 'spec_helper'
+require "spec_helper"
 
 describe SimpleHdGraph::Renderer::PlantUML::Context do
   include ExampleLoader
@@ -6,8 +6,8 @@
   let(:parser) { SimpleHdGraph::Parser.new }
   let(:renderer) { SimpleHdGraph::Renderer::PlantUML::Context.new }
 
-  describe '#render' do
-    describe 'simple' do
+  describe "#render" do
+    describe "simple" do
       before {
         @nodes = parser.parse(read_example(:simple))
       }
@@ -19,7 +19,7 @@
       }
     end
 
-    describe 'complex' do
+    describe "complex" do
       before {
         @nodes = parser.parse(read_example(:complex))
       }
@@ -31,7 +31,7 @@
       }
     end
 
-    describe 'depends' do
+    describe "depends" do
       before {
         @nodes = parser.parse(
           [
diff --git a/spec/renderer/plantuml/resource_spec.rb b/spec/renderer/plantuml/resource_spec.rb
index 4677903..73be0d8 100644
--- a/spec/renderer/plantuml/resource_spec.rb
+++ b/spec/renderer/plantuml/resource_spec.rb
@@ -1,4 +1,4 @@
-require 'spec_helper'
+require "spec_helper"
 
 describe SimpleHdGraph::Renderer::PlantUML::Resource do
   include ExampleLoader
@@ -6,7 +6,7 @@
   let(:parser) { SimpleHdGraph::Parser.new }
   let(:renderer) { SimpleHdGraph::Renderer::PlantUML::Resource.new }
 
-  describe '#render' do
+  describe "#render" do
     before {
       @nodes = parser.parse(read_example(:simple))
     }
diff --git a/spec/resource_spec.rb b/spec/resource_spec.rb
index 8a49173..ae61ac8 100644
--- a/spec/resource_spec.rb
+++ b/spec/resource_spec.rb
@@ -1,4 +1,4 @@
-require 'spec_helper'
+require "spec_helper"
 
 describe SimpleHdGraph::ResourceNode do
   include ExampleLoader
@@ -7,18 +7,18 @@
 
   # @return [Hash]
   def testing_context
-    YAML.load(<<EOD, symbolize_names: true)
-id: context
-EOD
+    YAML.safe_load(<<~EOD, symbolize_names: true)
+      id: context
+    EOD
   end
 
   # @return [Hash]
   def testing_resource
-    YAML.load(<<EOD, symbolize_names: true)
-web:
-  hosting: Heroku-16
-  runtime: Ruby 2.5
-EOD
+    YAML.safe_load(<<~EOD, symbolize_names: true)
+      web:
+        hosting: Heroku-16
+        runtime: Ruby 2.5
+    EOD
   end
 
   before {
@@ -27,37 +27,37 @@ def testing_resource
     @node.load_with_context(context, testing_resource)
   }
 
-  describe '#alias' do
+  describe "#alias" do
     it {
-      assert { @node.alias == 'web' }
+      assert { @node.alias == "web" }
     }
   end
 
-  describe '#id' do
+  describe "#id" do
     it {
-      assert { @node.id == 'contextWeb' }
+      assert { @node.id == "contextWeb" }
     }
   end
 
-  describe '#content' do
-    it 'does not include `has`' do
+  describe "#content" do
+    it "does not include `has`" do
       nodes = parser.parse(read_example(:complex))
 
       assert {
         nodes.first.resources.first.content == {
-          'hosting' => 'Heroku',
-          'runtime' => 'Ruby 2.5'
+          "hosting" => "Heroku",
+          "runtime" => "Ruby 2.5"
         }
       }
     end
   end
 
-  describe '#has' do
+  describe "#has" do
     it {
       nodes = parser.parse(read_example(:complex))
 
       assert {
-        nodes.first.resources.first.has == ['admin', 'storage']
+        nodes.first.resources.first.has == ["admin", "storage"]
       }
     }
   end
diff --git a/spec/simple-hd-graph_spec.rb b/spec/simple-hd-graph_spec.rb
index c8ac1c1..5f3772d 100644
--- a/spec/simple-hd-graph_spec.rb
+++ b/spec/simple-hd-graph_spec.rb
@@ -1,7 +1,7 @@
 require "spec_helper"
 
 describe SimpleHdGraph do
-  it 'just dummy for spec style and power-assert' do
+  it "just dummy for spec style and power-assert" do
     assert {
       SimpleHdGraph
     }
diff --git a/spec/spec_helper.rb b/spec/spec_helper.rb
index 8df6690..c9d6432 100644
--- a/spec/spec_helper.rb
+++ b/spec/spec_helper.rb
@@ -7,8 +7,8 @@
 
 Minitest::Reporters.use! Minitest::Reporters::SpecReporter.new
 
-Dir.glob(__dir__ + '/support/**/*.rb').select {|f|
+Dir.glob(__dir__ + "/support/**/*.rb").select { |f|
   f !~ /_(spec|test)\.rb/
-}.each {|f|
+}.each { |f|
   require f
 }
diff --git a/spec/support/example_loader.rb b/spec/support/example_loader.rb
index a2db453..71cd24a 100644
--- a/spec/support/example_loader.rb
+++ b/spec/support/example_loader.rb
@@ -7,11 +7,11 @@ class ExampleNotFound < StandardError; end
   #
   def read_example(name)
     examples = Dir.glob(__dir__ + "/example_#{name}.{yml,yaml}")
-    
+
     if examples.size > 0
       File.read(examples.first)
     else
       raise ExampleNotFound.new name
-    end    
+    end
   end
 end