diff --git a/test/ruby/test_m17n.rb b/test/ruby/test_m17n.rb
index 3423a1d58a..24b4dd2abb 100644
--- a/test/ruby/test_m17n.rb
+++ b/test/ruby/test_m17n.rb
@@ -653,7 +653,7 @@ class TestM17N < Test::Unit::TestCase
 
   def test_str_String
     s = String(10)
-    assert_equal(Encoding::ASCII_8BIT, s.encoding)
+    assert_equal(Encoding::US_ASCII, s.encoding)
   end
 
   def test_sprintf_c