diff --git a/ChangeLog b/ChangeLog
index 96435d3..307473c 100644
--- a/ChangeLog
+++ b/ChangeLog
@@ -1,5 +1,7 @@
 2014-12-12  Niibe Yutaka  <gniibe@fsij.org>
 
+	* src/Makefile.in (build/bignum.o): Specific OPT for this target.
+
 	* src/configure (target): Default is FST-01.
 	(--with-dfu): FST-01 doesn't have DFU.  If set, it must be
 	mistake.
diff --git a/src/Makefile.in b/src/Makefile.in
index e2e8e92..806af50 100644
--- a/src/Makefile.in
+++ b/src/Makefile.in
@@ -68,6 +68,8 @@ board.h:
 
 sys.c: board.h
 
+build/bignum.o: OPT = -O3 -g
+
 distclean: clean
 	-rm -f gnuk.ld config.h board.h Makefile \
 	       usb-strings.c.inc usb-vid-pid-ver.c.inc