diff --git a/ChangeLog b/ChangeLog
index 84debb3cef..75046c625b 100644
--- a/ChangeLog
+++ b/ChangeLog
@@ -1,3 +1,9 @@
+Mon Dec 10 10:36:12 2012  Narihiro Nakamura  <authornari@gmail.com>
+
+	* lib/irb/magic-file.rb: set a encoding, which is detected from
+	  the file to read, to the internal encoding.
+	  [Bug #4281][ruby-dev:43036]
+
 Mon Dec 10 09:40:19 2012  Eric Hodel  <drbrain@segment7.net>
 
 	* lib/rubygems/ext/cmake_builder.rb:  Added a builder for cmake.
diff --git a/lib/irb/magic-file.rb b/lib/irb/magic-file.rb
index 861262050e..339ed60b6b 100644
--- a/lib/irb/magic-file.rb
+++ b/lib/irb/magic-file.rb
@@ -8,9 +8,10 @@ module IRB
       line = io.gets
       line = io.gets if line[0,2] == "#!"
       encoding = detect_encoding(line)
+      internal_encoding = encoding
       encoding ||= default_src_encoding
       io.rewind
-      io.set_encoding(encoding, nil)
+      io.set_encoding(encoding, internal_encoding)
 
       if block_given?
         begin