diff --git a/ChangeLog b/ChangeLog
index 2a59f7c653..207630eb36 100644
--- a/ChangeLog
+++ b/ChangeLog
@@ -1,3 +1,8 @@
+Thu Nov 24 19:59:56 2011  Tanaka Akira  <akr@fsij.org>
+
+	* io.c (rb_io_reopen): re-initialize buffereing mode for stdout and
+	  stderr.
+
 Thu Nov 24 11:12:48 2011  KOSAKI Motohiro  <kosaki.motohiro@gmail.com>
 
 	* io.c (rb_io_fsync,rb_io_fdatasync): release GVL during fsync().
diff --git a/io.c b/io.c
index 31b07f9c0a..3f35c1982c 100644
--- a/io.c
+++ b/io.c
@@ -6151,6 +6151,14 @@ rb_io_reopen(int argc, VALUE *argv, VALUE file)
         if (setvbuf(fptr->stdio_file, NULL, _IOFBF, 0) != 0)
             rb_warn("setvbuf() can't be honoured for %s", RSTRING_PTR(fptr->pathv));
 #endif
+        if (fptr->stdio_file == stderr) {
+            if (setvbuf(fptr->stdio_file, NULL, _IONBF, BUFSIZ) != 0)
+                rb_warn("setvbuf() can't be honoured for %s", RSTRING_PTR(fptr->pathv));
+        }
+        else if (fptr->stdio_file == stdout && isatty(fptr->fd)) {
+            if (setvbuf(fptr->stdio_file, NULL, _IOLBF, BUFSIZ) != 0)
+                rb_warn("setvbuf() can't be honoured for %s", RSTRING_PTR(fptr->pathv));
+        }
     }
     else {
         if (close(fptr->fd) < 0)