diff --git a/.github/workflows/build.yml b/.github/workflows/build.yml
index 177bb23..54c18e0 100644
--- a/.github/workflows/build.yml
+++ b/.github/workflows/build.yml
@@ -42,6 +42,7 @@ jobs:
           - '2.6'
           - '2.7'
           - '3.0'
+          - '3.1'
         gemfile:
           - rails50.gemfile
           - rails51.gemfile
@@ -63,6 +64,12 @@ jobs:
             gemfile: rails51.gemfile
           - ruby: '3.0'
             gemfile: rails52.gemfile
+          - ruby: '3.1'
+            gemfile: rails50.gemfile
+          - ruby: '3.1'
+            gemfile: rails51.gemfile
+          - ruby: '3.1'
+            gemfile: rails52.gemfile
 
     env:
       CI: true
diff --git a/CHANGELOG.md b/CHANGELOG.md
index d27ee51..f07e1df 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -1,5 +1,9 @@
 ## Unreleased
 
+### Features
+
+* Ruby 3.1 をサポートに追加 (PR [#54](https://github.com/chocoby/jp_prefecture/pull/54)/[@chocoby](https://github.com/chocoby))
+
 ## 1.0.0 (2021-02-24)
 
 ### Misc
diff --git a/README.md b/README.md
index 90ce5c0..4bd7b6c 100644
--- a/README.md
+++ b/README.md
@@ -210,7 +210,7 @@ $ gem install jp_prefecture
 
 ## サポートしているバージョン
 
-* Ruby: 2.4 - 3.0
+* Ruby: 2.4 - 3.1
 * Rails: 5.0 - 6.1
 
 これより古い Ruby/Rails バージョンを使用する場合は、[`v0.11.0`](https://github.com/chocoby/jp_prefecture/tree/0.x) を利用してください。
diff --git a/README_EN.md b/README_EN.md
index cc9227e..3d98e5c 100644
--- a/README_EN.md
+++ b/README_EN.md
@@ -213,7 +213,7 @@ $ gem install jp_prefecture
 
 ## Supported versions
 
-* Ruby: 2.4 - 3.0
+* Ruby: 2.4 - 3.1
 * Rails: 5.0 - 6.1
 
 If you are using an older Ruby/Rails version, please use [`v0.11.0`](https://github.com/chocoby/jp_prefecture/tree/0.x).