diff --git a/Gemfile b/Gemfile
index 6d789a8..efd7637 100644
--- a/Gemfile
+++ b/Gemfile
@@ -1,6 +1,6 @@
 source 'https://rubygems.org'
 
-# Specify your gem's dependencies in gimei.gemspec
 gemspec
 gem 'simplecov', require: false
 gem 'steep', require: false
+gem 'minitest-mock' if Gem::Version.new(RUBY_VERSION) >= Gem::Version.new('3.1')
diff --git a/spec/spec_helper.rb b/spec/spec_helper.rb
index 3ae5e6f..9389928 100644
--- a/spec/spec_helper.rb
+++ b/spec/spec_helper.rb
@@ -6,6 +6,7 @@
 SimpleCov.start
 require 'gimei'
 require 'minitest/autorun'
+require 'minitest/mock'
 
 def zenkaku_regexp
   /\p{Hiragana}|\p{Katakana}|[一-龠々]/
@@ -14,4 +15,3 @@ def zenkaku_regexp
 def zenkaku_or_space_regexp
   /\p{Hiragana}|\p{Katakana}|[一-龠々]|\s/
 end
-