diff --git a/ChangeLog b/ChangeLog
index 1ed85711a8..10b9fa22e5 100644
--- a/ChangeLog
+++ b/ChangeLog
@@ -1,3 +1,7 @@
+Sun Oct 18 08:50:15 2015  KOSAKI Motohiro  <kosaki.motohiro@gmail.com>
+
+	* include/ruby/defines.h (DOSISH): add comments.
+
 Sun Oct 18 08:26:51 2015  KOSAKI Motohiro  <kosaki.motohiro@gmail.com>
 
 	* io.c (fptr_finalize): don't release gvl if fptr is not writable.
diff --git a/include/ruby/defines.h b/include/ruby/defines.h
index 6dfb44e407..88ab53f8e6 100644
--- a/include/ruby/defines.h
+++ b/include/ruby/defines.h
@@ -148,6 +148,11 @@ void xfree(void*);
 #endif
 
 #if defined(_WIN32) || defined(__EMX__)
+/*
+  DOSISH mean MS-Windows style filesystem.
+  But you should use more precise macros like DOSISH_DRIVE_LETTER, PATH_SEP,
+  ENV_IGNORECASE or CASEFOLD_FILESYSTEM.
+ */
 #define DOSISH 1
 # define DOSISH_DRIVE_LETTER
 #endif