diff --git a/.github/workflows/ci.yml b/.github/workflows/ci.yml
index 83a5fd2..6a26232 100644
--- a/.github/workflows/ci.yml
+++ b/.github/workflows/ci.yml
@@ -21,3 +21,55 @@ jobs:
           skip_install: 'true'
           use_bundler: 'true'
       - run: test "$(bundle exec rubocop --version)" == "1.18.1"
+  test-only_changed:
+    runs-on: ubuntu-latest
+    defaults:
+      run:
+        shell: bash
+    env:
+      INPUT_ONLY_CHANGED: 'true'
+    steps:
+      - uses: actions/checkout@v4
+      - name: setup
+        run: |
+          git config user.email "workflow@github.com"
+          git config user.name "I am an automated workflow"
+      - name: Check when there are relevant files
+        run: |
+          git checkout ${{ github.sha }}
+          rm -f test/only_changed/reviewdog-was-called
+
+          cp test/only_changed/few_relevant/files/* .
+          git add *
+          git commit -m auto
+
+          export PATH=test/only_changed/few_relevant/mock_bins:test/only_changed/shared_mock_bins:$PATH
+          BASE_REF=$(git rev-parse HEAD~) HEAD_REF=$(git rev-parse HEAD) ./script.sh
+
+          [ -f test/only_changed/reviewdog-was-called ]
+      - name: Check when there are no relevant files
+        run: |
+          git checkout ${{ github.sha }}
+          rm -f test/only_changed/reviewdog-was-called
+
+          cp test/only_changed/nothing_relevant/files/* .
+          git add *
+          git commit -m auto
+
+          export PATH=test/only_changed/nothing_relevant/mock_bins:test/only_changed/shared_mock_bins:$PATH
+          BASE_REF=$(git rev-parse HEAD~) HEAD_REF=$(git rev-parse HEAD) ./script.sh
+
+          [ ! -f test/only_changed/reviewdog-was-called ]
+      - name: Check when there are too many relevant files
+        run: |
+          git checkout ${{ github.sha }}
+          rm -f test/only_changed/reviewdog-was-called
+
+          touch a{00..100}.rb
+          git add *
+          git commit -m auto
+
+          export PATH=test/only_changed/too_many_relevant/mock_bins:test/only_changed/shared_mock_bins:$PATH
+          BASE_REF=$(git rev-parse HEAD~) HEAD_REF=$(git rev-parse HEAD) ./script.sh
+
+          [ -f test/only_changed/reviewdog-was-called ]
diff --git a/test/only_changed/few_relevant/files/a.rb b/test/only_changed/few_relevant/files/a.rb
new file mode 100644
index 0000000..5460224
--- /dev/null
+++ b/test/only_changed/few_relevant/files/a.rb
@@ -0,0 +1 @@
+puts "Hello, " + "world!"
diff --git a/test/only_changed/few_relevant/files/a.txt b/test/only_changed/few_relevant/files/a.txt
new file mode 100644
index 0000000..5460224
--- /dev/null
+++ b/test/only_changed/few_relevant/files/a.txt
@@ -0,0 +1 @@
+puts "Hello, " + "world!"
diff --git a/test/only_changed/few_relevant/mock_bins/rubocop b/test/only_changed/few_relevant/mock_bins/rubocop
new file mode 100755
index 0000000..40c1a97
--- /dev/null
+++ b/test/only_changed/few_relevant/mock_bins/rubocop
@@ -0,0 +1,10 @@
+#!/usr/bin/env ruby
+
+case ARGV
+when %w[--list-target-files]
+  puts Dir['**/*.rb']
+when %W[--require #{ENV['GITHUB_ACTION_PATH']}/rdjson_formatter/rdjson_formatter.rb --format RdjsonFormatter --fail-level error a.rb]
+  puts 'Mock message for reviewdog'
+else
+  abort "rubocop mock called with unexpected arguments:\n#{ARGV.join("\n")}"
+end
diff --git a/test/only_changed/nothing_relevant/files/a.txt b/test/only_changed/nothing_relevant/files/a.txt
new file mode 100644
index 0000000..5460224
--- /dev/null
+++ b/test/only_changed/nothing_relevant/files/a.txt
@@ -0,0 +1 @@
+puts "Hello, " + "world!"
diff --git a/test/only_changed/nothing_relevant/mock_bins/rubocop b/test/only_changed/nothing_relevant/mock_bins/rubocop
new file mode 100755
index 0000000..36ab0d3
--- /dev/null
+++ b/test/only_changed/nothing_relevant/mock_bins/rubocop
@@ -0,0 +1,8 @@
+#!/usr/bin/env ruby
+
+case ARGV
+when %w[--list-target-files]
+  puts Dir['**/*.rb']
+else
+  abort "rubocop mock called with unexpected arguments:\n#{ARGV.join("\n")}"
+end
diff --git a/test/only_changed/shared_mock_bins/bundle b/test/only_changed/shared_mock_bins/bundle
new file mode 100755
index 0000000..0a28632
--- /dev/null
+++ b/test/only_changed/shared_mock_bins/bundle
@@ -0,0 +1,9 @@
+#!/bin/bash
+
+if [ "$1" = "exec" ]; then
+  shift
+  eval "$@"
+else
+  echo "Only 'exec' command is supported"
+  exit 1
+fi
diff --git a/test/only_changed/shared_mock_bins/curl b/test/only_changed/shared_mock_bins/curl
new file mode 100755
index 0000000..3c5bc23
--- /dev/null
+++ b/test/only_changed/shared_mock_bins/curl
@@ -0,0 +1,8 @@
+#!/bin/bash
+
+arguments="$*"
+
+if [ "$arguments" != "-sfL https://raw.githubusercontent.com/reviewdog/reviewdog/master/install.sh" ]; then
+  echo "curl mock got unexpected arguments: $arguments"
+  exit 1
+fi
diff --git a/test/only_changed/shared_mock_bins/reviewdog b/test/only_changed/shared_mock_bins/reviewdog
new file mode 100755
index 0000000..5334ef7
--- /dev/null
+++ b/test/only_changed/shared_mock_bins/reviewdog
@@ -0,0 +1,10 @@
+#!/bin/bash
+
+touch test/only_changed/reviewdog-was-called
+
+read -r input
+
+if [ "$input" != "Mock message for reviewdog" ]; then
+  echo "reviewdog mock got unexpected input: $input"
+  exit 1
+fi
diff --git a/test/only_changed/too_many_relevant/mock_bins/rubocop b/test/only_changed/too_many_relevant/mock_bins/rubocop
new file mode 100755
index 0000000..cfab6d4
--- /dev/null
+++ b/test/only_changed/too_many_relevant/mock_bins/rubocop
@@ -0,0 +1,10 @@
+#!/usr/bin/env ruby
+
+case ARGV
+when %w[--list-target-files]
+  puts Dir['**/*.rb']
+when %W[--require #{ENV['GITHUB_ACTION_PATH']}/rdjson_formatter/rdjson_formatter.rb --format RdjsonFormatter --fail-level error]
+  puts 'Mock message for reviewdog'
+else
+  abort "rubocop mock called with unexpected arguments:\n#{ARGV.join("\n")}"
+end