diff --git a/test/ruby/test_m17n.rb b/test/ruby/test_m17n.rb
index 3bfb540b13..d8f45078bf 100644
--- a/test/ruby/test_m17n.rb
+++ b/test/ruby/test_m17n.rb
@@ -1,5 +1,5 @@
 require 'test/unit'
-require 'stringio'
+require_relative 'envutil'
 
 class TestM17N < Test::Unit::TestCase
   def assert_encoding(encname, actual, message=nil)
@@ -23,17 +23,6 @@ class TestM17N < Test::Unit::TestCase
     assert_equal(a(bytes), a(actual), message)
   end
 
-  def assert_warning(pat, mesg=nil)
-    begin
-      org_stderr = $stderr
-      $stderr = StringIO.new(warn = '')
-      yield
-    ensure
-      $stderr = org_stderr
-    end
-    assert_match(pat, warn, mesg)
-  end
-
   def assert_regexp_generic_encoding(r)
     assert(!r.fixed_encoding?)
     %w[ASCII-8BIT EUC-JP Windows-31J UTF-8].each {|ename|