diff --git a/ChangeLog b/ChangeLog
index 261d426c90..0c54ae66f8 100644
--- a/ChangeLog
+++ b/ChangeLog
@@ -1,4 +1,9 @@
-Mon Jul  1 15:04:20 2013  Nobuyoshi Nakada  <nobu@ruby-lang.org>
+Mon Jul  1 15:05:00 2013  Nobuyoshi Nakada  <nobu@ruby-lang.org>
+
+	* lib/csv.rb (CSV#<<): use StringIO#set_encoding instead of creating
+	  new StringIO instance with String#force_encoding, forcing encoding
+	  discards the cached coderange bits and can make further operations
+	  very slow.  [ruby-core:55714] [Bug #8585]
 
 	* ext/stringio/stringio.c (strio_write): keep coderange of
 	  ptr->string.
diff --git a/lib/csv.rb b/lib/csv.rb
index fe153153be..b6d9121146 100644
--- a/lib/csv.rb
+++ b/lib/csv.rb
@@ -1655,7 +1655,7 @@ class CSV
     if @io.is_a?(StringIO)             and
        output.encoding != raw_encoding and
        (compatible_encoding = Encoding.compatible?(@io.string, output))
-      @io = StringIO.new(@io.string.force_encoding(compatible_encoding))
+      @io.set_encoding(compatible_encoding)
       @io.seek(0, IO::SEEK_END)
     end
     @io << output