diff --git a/tool/lib/test/unit.rb b/tool/lib/test/unit.rb
index 45c751033b..fd3b19406c 100644
--- a/tool/lib/test/unit.rb
+++ b/tool/lib/test/unit.rb
@@ -2,9 +2,9 @@
 
 require_relative '../envutil'
 require_relative '../colorize'
-require 'test/unit/testcase'
+require_relative '../leakchecker'
+require_relative '../test/unit/testcase'
 require 'optparse'
-require "leakchecker"
 
 # See Test::Unit
 module Test
diff --git a/tool/lib/test/unit/parallel.rb b/tool/lib/test/unit/parallel.rb
index 47ff0b360a..96aadf0249 100644
--- a/tool/lib/test/unit/parallel.rb
+++ b/tool/lib/test/unit/parallel.rb
@@ -1,12 +1,12 @@
 # frozen_string_literal: true
-$LOAD_PATH.unshift "#{File.dirname(__FILE__)}/../.."
-require 'test/unit'
+$LOAD_PATH.unshift "#{__dir__}/../.."
+require_relative '../../test/unit'
 
-require "profile_test_all" if ENV.key?('RUBY_TEST_ALL_PROFILE')
-require "tracepointchecker"
-require "zombie_hunter"
-require "iseq_loader_checker"
-require "gc_checker"
+require_relative '../../profile_test_all' if ENV.key?('RUBY_TEST_ALL_PROFILE')
+require_relative '../../tracepointchecker'
+require_relative '../../zombie_hunter'
+require_relative '../../iseq_loader_checker'
+require_relative '../../gc_checker'
 
 module Test
   module Unit
diff --git a/tool/lib/test/unit/testcase.rb b/tool/lib/test/unit/testcase.rb
index 4cc1aae3e4..13256f8ed1 100644
--- a/tool/lib/test/unit/testcase.rb
+++ b/tool/lib/test/unit/testcase.rb
@@ -1,5 +1,5 @@
 # frozen_string_literal: true
-require 'test/unit/assertions'
+require_relative 'assertions'
 require_relative '../../core_assertions'
 
 module Test