diff --git a/ChangeLog b/ChangeLog
index 7f65227b7e..e86065a29a 100644
--- a/ChangeLog
+++ b/ChangeLog
@@ -1,3 +1,7 @@
+Sat May 31 23:33:34 2008  Akinori MUSHA  <knu@iDaemons.org>
+
+	* README, README.ja: Add a note about default C flags.
+
 Sat May 31 22:11:15 2008  Kazuhiro NISHIYAMA  <zn@mbf.nifty.com>
 
 	* version.c (ruby_description, ruby_copyright): backported from
diff --git a/README b/README
index df0d6e701d..1b79781367 100644
--- a/README
+++ b/README
@@ -70,6 +70,10 @@ This is what you need to do to compile and install Ruby:
 
   2. Run ./configure, which will generate config.h and Makefile.
 
+     Some C compiler flags may be added by default depending on your
+     environment.  Specify optflags=.. and warnflags=.. as necessary
+     to override them.
+
   3. Edit defines.h if you need.  Usually this step will not be needed.
 
   4. Remove comment mark(#) before the module names from ext/Setup (or
diff --git a/README.ja b/README.ja
index 6a89d2a763..71ce4ee12c 100644
--- a/README.ja
+++ b/README.ja
@@ -92,6 +92,10 @@ Ruby
 
   2. configureを実行してMakefileなどを生成する
 
+     環境によってはデフォルトのCコンパイラ用オプションが付き
+     ます．configureオプションで optflags=.. warnflags=.. 等
+     で上書きできます．
+
   3. (必要ならば)defines.hを編集する
 
      多分，必要無いと思います．