diff --git a/ChangeLog b/ChangeLog
index ea5dd07028..5d51cf157f 100644
--- a/ChangeLog
+++ b/ChangeLog
@@ -1,3 +1,9 @@
+Mon Feb 27 20:43:05 2012  NAKAMURA Usaku  <usa@ruby-lang.org>
+
+	* io.c (rb_io_set_pos): add rdoc about textmode.
+
+	* test/ruby/test_io.rb (TestIO#test_setpos): use binmode.
+
 Mon Feb 27 17:00:15 2012  Akinori MUSHA  <knu@iDaemons.org>
 
 	* string.c (rb_str_crypt): Update rdoc and state that this
diff --git a/io.c b/io.c
index ab1e6e1e1c..f6375cd8db 100644
--- a/io.c
+++ b/io.c
@@ -1437,6 +1437,8 @@ rb_io_seek_m(int argc, VALUE *argv, VALUE io)
  *     ios.pos = integer    -> integer
  *
  *  Seeks to the given position (in bytes) in <em>ios</em>.
+ *  It is not guranteed that seeking to the right position when <em>ios</em>
+ *  is textmode.
  *
  *     f = File.new("testfile")
  *     f.pos = 17