diff --git a/.github/workflows/go-test.yml b/.github/workflows/go-test.yml
index 3482cef..39f2240 100644
--- a/.github/workflows/go-test.yml
+++ b/.github/workflows/go-test.yml
@@ -12,7 +12,7 @@ on:
         type: string
       go-versions:
         description: 'Stringfied JSON object listing target Go versions'
-        default: '["1.20.x", "1.19.x"]'
+        default: '["1.21.x", "1.20.x"]'
         required: false
         type: string
       pre:
diff --git a/.github/workflows/goreleaser.yml b/.github/workflows/goreleaser.yml
index 272a292..f88194d 100644
--- a/.github/workflows/goreleaser.yml
+++ b/.github/workflows/goreleaser.yml
@@ -4,6 +4,9 @@ on:
   workflow_call:
     inputs:
       go-version:
+        # TODO: upgrade default Go version after 2023-11-01.
+        # Go 1.21 requires Windows Server 2016 or higher.
+        # But Mackerel supports older Windows Servers yet.
         default: '1.20'
         required: false
         type: string
diff --git a/.github/workflows/setup-go-matrix.yml b/.github/workflows/setup-go-matrix.yml
index 6d83723..440a128 100644
--- a/.github/workflows/setup-go-matrix.yml
+++ b/.github/workflows/setup-go-matrix.yml
@@ -13,7 +13,7 @@ on:
         type: string
       go-versions:
         description: 'Stringfied JSON object listing target Go versions'
-        default: '["1.20.x", "1.19.x"]'
+        default: '["1.21.x", "1.20.x"]'
         required: false
         type: string
       run: