diff --git a/ChangeLog b/ChangeLog
index 646b4c55b1..853c5863dc 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 23:02:00 2008  Tanaka Akira  <akr@fsij.org>
 
 	* gc.c (count_objects): clear given hash.
diff --git a/README b/README
index 938c846e3f..b898003937 100644
--- a/README
+++ b/README
@@ -65,6 +65,10 @@ 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 ee1e0d4951..16dbd9368d 100644
--- a/README.ja
+++ b/README.ja
@@ -88,6 +88,10 @@ 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．