diff --git a/Rakefile b/Rakefile
index 65bb8dd..24b1b37 100644
--- a/Rakefile
+++ b/Rakefile
@@ -5,6 +5,8 @@ end
 
 require "rake/testtask"
 Rake::TestTask.new(:test) do |t|
+  t.libs << "test/lib"
+  t.ruby_opts << "-rhelper"
   t.test_files = FileList["test/**/test_*.rb"]
 end
 
diff --git a/test/lib/helper.rb b/test/lib/helper.rb
new file mode 100644
index 0000000..62a7542
--- /dev/null
+++ b/test/lib/helper.rb
@@ -0,0 +1,5 @@
+require 'test/unit'
+# for standalone test suite on ruby/logger
+require_relative 'core_assertions'
+
+Test::Unit::TestCase.include Test::Unit::CoreAssertions
diff --git a/test/logger/helper.rb b/test/logger/helper.rb
deleted file mode 100644
index 9eaeb20..0000000
--- a/test/logger/helper.rb
+++ /dev/null
@@ -1,13 +0,0 @@
-ROOT_DIR = File.dirname(__dir__)
-$LOAD_PATH.unshift File.join(ROOT_DIR, 'lib') # to use logger in this repo instead of ruby built-in logger
-$LOAD_PATH.unshift File.join(ROOT_DIR, 'test', 'lib') # to use custom test-unit in this repo
-require 'logger'
-require 'test/unit'
-
-begin
-  # for standalone test suite on ruby/logger
-  require 'core_assertions'
-
-  Test::Unit::TestCase.include Test::Unit::CoreAssertions
-rescue LoadError
-end
diff --git a/test/logger/test_logdevice.rb b/test/logger/test_logdevice.rb
index 6fefd35..d360fa2 100644
--- a/test/logger/test_logdevice.rb
+++ b/test/logger/test_logdevice.rb
@@ -1,6 +1,6 @@
 # coding: US-ASCII
 # frozen_string_literal: false
-require_relative 'helper'
+require 'logger'
 require 'tempfile'
 require 'tmpdir'
 
diff --git a/test/logger/test_logger.rb b/test/logger/test_logger.rb
index 4bbfd52..3281d17 100644
--- a/test/logger/test_logger.rb
+++ b/test/logger/test_logger.rb
@@ -1,6 +1,6 @@
 # coding: US-ASCII
 # frozen_string_literal: false
-require_relative 'helper'
+require 'logger'
 require 'tempfile'
 
 class TestLogger < Test::Unit::TestCase
diff --git a/test/logger/test_logperiod.rb b/test/logger/test_logperiod.rb
index 3c5cbbc..6e6e5e9 100644
--- a/test/logger/test_logperiod.rb
+++ b/test/logger/test_logperiod.rb
@@ -1,6 +1,6 @@
 # coding: US-ASCII
 # frozen_string_literal: false
-require_relative 'helper'
+require 'logger'
 require 'time'
 
 class TestLogPeriod < Test::Unit::TestCase
diff --git a/test/logger/test_severity.rb b/test/logger/test_severity.rb
index 1197e8a..dad6347 100644
--- a/test/logger/test_severity.rb
+++ b/test/logger/test_severity.rb
@@ -1,6 +1,6 @@
 # coding: US-ASCII
 # frozen_string_literal: false
-require_relative 'helper'
+require 'logger'
 
 class TestLoggerSeverity < Test::Unit::TestCase
   def test_enum