diff --git a/.github/workflows/test.yml b/.github/workflows/test.yml
index c994489..3761929 100644
--- a/.github/workflows/test.yml
+++ b/.github/workflows/test.yml
@@ -12,7 +12,7 @@ jobs:
     name: build (${{ matrix.ruby }} / ${{ matrix.os }})
     strategy:
       matrix:
-        ruby: [ '3.1', '3.0', 2.7, head ]
+        ruby: [ head, '3.1', '3.0', 2.7, 2.6 ]
         os: [ ubuntu-latest, macos-latest, windows-latest ]
         exclude:
           - ruby: head
diff --git a/date.gemspec b/date.gemspec
index 2bf28ff..660353e 100644
--- a/date.gemspec
+++ b/date.gemspec
@@ -25,7 +25,7 @@ Gem::Specification.new do |s|
     s.extensions = "ext/date/extconf.rb"
   end
 
-  s.required_ruby_version = ">= 2.4.0"
+  s.required_ruby_version = ">= 2.6.0"
 
   s.authors = ["Tadayoshi Funaba"]
   s.email = [nil]