diff --git a/test/ruby/test_io_m17n.rb b/test/ruby/test_io_m17n.rb
index 1087d182ae..6bc9025f1a 100644
--- a/test/ruby/test_io_m17n.rb
+++ b/test/ruby/test_io_m17n.rb
@@ -48,7 +48,17 @@ EOT
     with_tmpdir {
       generate_file('tmp', "")
       open("tmp", "r") {|f|
-        assert_equal(nil, f.external_encoding)
+        assert_equal(Encoding.default_external, f.external_encoding)
+        assert_equal(nil, f.internal_encoding)
+      }
+    }
+  end
+
+  def test_open_rb
+    with_tmpdir {
+      generate_file('tmp', "")
+      open("tmp", "rb") {|f|
+        assert_equal(Encoding::ASCII_8BIT, f.external_encoding)
         assert_equal(nil, f.internal_encoding)
       }
     }
@@ -83,6 +93,15 @@ EOT
     }
   end
 
+  def test_open_wb
+    with_tmpdir {
+      open("tmp", "wb") {|f|
+        assert_equal(nil, f.external_encoding)
+        assert_equal(nil, f.internal_encoding)
+      }
+    }
+  end
+
   def test_open_w_enc
     with_tmpdir {
       open("tmp", "w:euc-jp") {|f|