diff --git a/.github/workflows/test-ruby.yml b/.github/workflows/test-ruby.yml
index 674fb93..10f46f2 100644
--- a/.github/workflows/test-ruby.yml
+++ b/.github/workflows/test-ruby.yml
@@ -5,7 +5,9 @@ on:
     paths:
       - '.github/workflows/test-ruby.yml'
       - 'ruby/lib/**'
+      - 'ruby/spec/**'
       - 'ruby/Gemfile'
+      - 'ruby/Gemfile.lock'
 
 concurrency:
   group: ${{ github.workflow }}-${{ github.ref }}
@@ -17,8 +19,11 @@ permissions:
 jobs:
   build:
     runs-on: ubuntu-latest
-    name: Build
+    name: Build (Ruby ${{ matrix.ruby-version }})
     timeout-minutes: 5
+    strategy:
+      matrix:
+        ruby-version: ['3.2', '3.3', '3.4', '4.0']
     defaults:
       run:
         working-directory: ruby
@@ -28,9 +33,9 @@ jobs:
       - uses: actions/checkout@de0fac2e4500dabe0009e67214ff5f5447ce83dd # v6
         with:
           persist-credentials: false
-      - uses: ruby/setup-ruby@354a1ad156761f5ee2b7b13fa8e09943a5e8d252 # v1
+      - uses: ruby/setup-ruby@95ef2b042f9d7a56d8268cba8559e2842e2ad01b # v1
         with:
-          ruby-version: '3.3'
+          ruby-version: ${{ matrix.ruby-version }}
           bundler-cache: true
           working-directory: ruby
       - run: find lib -name '*.rb' -exec ruby -c {} +
diff --git a/ruby/Gemfile b/ruby/Gemfile
index d3f73fa..8f677e1 100644
--- a/ruby/Gemfile
+++ b/ruby/Gemfile
@@ -1,5 +1,7 @@
 source "https://rubygems.org"
 
+gem "bigdecimal", "~> 4.1"
+
 group :test do
   gem "rspec", "~> 3.13"
 end
diff --git a/ruby/Gemfile.lock b/ruby/Gemfile.lock
index 116795e..4f3ea37 100644
--- a/ruby/Gemfile.lock
+++ b/ruby/Gemfile.lock
@@ -1,6 +1,7 @@
 GEM
   remote: https://rubygems.org/
   specs:
+    bigdecimal (4.1.2)
     diff-lcs (1.6.2)
     rspec (3.13.2)
       rspec-core (~> 3.13.0)
@@ -20,7 +21,8 @@ PLATFORMS
   ruby
 
 DEPENDENCIES
+  bigdecimal (~> 4.1)
   rspec (~> 3.13)
 
 BUNDLED WITH
-   2.5.6
+   2.7.2
diff --git a/ruby/README.md b/ruby/README.md
index 11b5540..10f817c 100644
--- a/ruby/README.md
+++ b/ruby/README.md
@@ -2,7 +2,7 @@
 
 ## 開発
 
-- Ruby 3.0+
+- Ruby 3.2+ (or Ruby 4.0+)
 
 ```shell
 # 準備
@@ -11,7 +11,8 @@ git add .
 git commit -m init
 
 # setup
-bundle install --path vendor/bundle
+bundle config set --local path vendor/bundle
+bundle install
 
 # test
 bundle exec rspec