diff --git a/ChangeLog b/ChangeLog
index e01f5a70ff..f5c5b81e65 100644
--- a/ChangeLog
+++ b/ChangeLog
@@ -1,3 +1,9 @@
+Wed May 15 00:42:54 2013  NAKAMURA Usaku  <usa@ruby-lang.org>
+
+	* signal.c: need to include unistd.h for write(2).
+	  unistd.h is now included via ruby/defines.h, but should implicitly
+	  include here.  (suggested by kosaki)
+
 Tue May 14 23:43:05 2013  Tanaka Akira  <akr@fsij.org>
 
 	* ext/socket/.document: Add ifaddr.c.
diff --git a/signal.c b/signal.c
index 7b58840d28..47c096b24c 100644
--- a/signal.c
+++ b/signal.c
@@ -19,6 +19,9 @@
 #include "ruby_atomic.h"
 #include "eval_intern.h"
 #include "internal.h"
+#ifdef HAVE_UNISTD_H
+# include <unistd.h>
+#endif
 
 #if defined(__native_client__) && defined(NACL_NEWLIB)
 # include "nacl/signal.h"