diff --git a/.github/workflows/build.yml b/.github/workflows/build.yml
index 4d9f5dc..dd95eea 100644
--- a/.github/workflows/build.yml
+++ b/.github/workflows/build.yml
@@ -44,6 +44,7 @@ jobs:
           - '3.2'
           - '3.3'
           - '3.4'
+          - '4.0'
         gemfile:
           - rails-5-0.gemfile
           - rails-5-1.gemfile
@@ -67,6 +68,8 @@ jobs:
             gemfile: rails-5-0.gemfile
           - ruby: '3.4'
             gemfile: rails-5-0.gemfile
+          - ruby: '4.0'
+            gemfile: rails-5-0.gemfile
 
           - ruby: '3.0'
             gemfile: rails-5-1.gemfile
@@ -78,6 +81,8 @@ jobs:
             gemfile: rails-5-1.gemfile
           - ruby: '3.4'
             gemfile: rails-5-1.gemfile
+          - ruby: '4.0'
+            gemfile: rails-5-1.gemfile
 
           - ruby: '3.0'
             gemfile: rails-5-2.gemfile
@@ -89,6 +94,8 @@ jobs:
             gemfile: rails-5-2.gemfile
           - ruby: '3.4'
             gemfile: rails-5-2.gemfile
+          - ruby: '4.0'
+            gemfile: rails-5-2.gemfile
 
           # Rails 6
           - ruby: '2.4'
diff --git a/CHANGELOG.md b/CHANGELOG.md
index debe1b0..b7b8d60 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -3,6 +3,7 @@
 ### Features
 
 * Rails 8.1 をサポートに追加 (PR [#78](https://github.com/chocoby/jp_prefecture/pull/78)/[@chocoby](https://github.com/chocoby))
+* Ruby 4.0 をサポートに追加 (PR [#80](https://github.com/chocoby/jp_prefecture/pull/80)/[@chocoby](https://github.com/chocoby))
 
 ## 1.1.2 (2025-02-24)
 
diff --git a/CONTRIBUTING.md b/CONTRIBUTING.md
index d436190..18f29f0 100644
--- a/CONTRIBUTING.md
+++ b/CONTRIBUTING.md
@@ -28,6 +28,14 @@ BUNDLE_GEMFILE=gemfiles/rails-5-1.gemfile bundle exec rspec
 BUNDLE_GEMFILE=gemfiles/rails-5-2.gemfile bundle install
 BUNDLE_GEMFILE=gemfiles/rails-5-2.gemfile bundle exec rspec
 
+# Rails 6.0
+BUNDLE_GEMFILE=gemfiles/rails-6-0.gemfile bundle install
+BUNDLE_GEMFILE=gemfiles/rails-6-0.gemfile bundle exec rspec
+
+# Rails 6.1
+BUNDLE_GEMFILE=gemfiles/rails-6-1.gemfile bundle install
+BUNDLE_GEMFILE=gemfiles/rails-6-1.gemfile bundle exec rspec
+
 # Rails 7.0
 BUNDLE_GEMFILE=gemfiles/rails-7-0.gemfile bundle install
 BUNDLE_GEMFILE=gemfiles/rails-7-0.gemfile bundle exec rspec
@@ -43,6 +51,10 @@ BUNDLE_GEMFILE=gemfiles/rails-7-2.gemfile bundle exec rspec
 # Rails 8.0
 BUNDLE_GEMFILE=gemfiles/rails-8-0.gemfile bundle install
 BUNDLE_GEMFILE=gemfiles/rails-8-0.gemfile bundle exec rspec
+
+# Rails 8.1
+BUNDLE_GEMFILE=gemfiles/rails-8-1.gemfile bundle install
+BUNDLE_GEMFILE=gemfiles/rails-8-1.gemfile bundle exec rspec
 ```
 
 ## Lint の実行
diff --git a/README.md b/README.md
index 4962a2b..096d344 100644
--- a/README.md
+++ b/README.md
@@ -216,7 +216,7 @@ $ gem install jp_prefecture
 
 ## サポートしているバージョン
 
-* Ruby: 2.4 - 3.4
+* Ruby: 2.4 - 4.0
 * Rails: 5.0 - 8.1
 
 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 3e5cef6..76156bc 100644
--- a/README_EN.md
+++ b/README_EN.md
@@ -219,7 +219,7 @@ $ gem install jp_prefecture
 
 ## Supported versions
 
-* Ruby: 2.4 - 3.4
+* Ruby: 2.4 - 4.0
 * Rails: 5.0 - 8.1
 
 If you are using an older Ruby/Rails version, please use [`v0.11.0`](https://github.com/chocoby/jp_prefecture/tree/0.x).
diff --git a/gemfiles/rails-6-0.gemfile b/gemfiles/rails-6-0.gemfile
index 776c6b7..f4ed056 100644
--- a/gemfiles/rails-6-0.gemfile
+++ b/gemfiles/rails-6-0.gemfile
@@ -4,6 +4,8 @@ eval_gemfile '../Gemfile'
 
 gem 'activerecord', '~> 6.0'
 gem 'base64'
+gem 'benchmark'
 gem 'bigdecimal'
+gem 'logger'
 gem 'mutex_m'
 gem 'sqlite3', '~> 1.4'
diff --git a/gemfiles/rails-6-1.gemfile b/gemfiles/rails-6-1.gemfile
index 4bbde6d..9066ac9 100644
--- a/gemfiles/rails-6-1.gemfile
+++ b/gemfiles/rails-6-1.gemfile
@@ -4,6 +4,8 @@ eval_gemfile '../Gemfile'
 
 gem 'activerecord', '~> 6.1'
 gem 'base64'
+gem 'benchmark'
 gem 'bigdecimal'
+gem 'logger'
 gem 'mutex_m'
 gem 'sqlite3', '~> 1.4'