diff --git a/ChangeLog b/ChangeLog
index 4c555d39aa..bcd72e5369 100644
--- a/ChangeLog
+++ b/ChangeLog
@@ -1,3 +1,7 @@
+Sun Jul 14 00:17:42 2013  Tanaka Akira  <akr@fsij.org>
+
+	* bignum.c (big_shift): Avoid signed integer overflow.
+
 Sun Jul 14 00:14:15 2013  Tanaka Akira  <akr@fsij.org>
 
 	* bignum.c (bary_mul_precheck): Use bary_small_lshift or
diff --git a/bignum.c b/bignum.c
index a20f8369fd..fb3954624c 100644
--- a/bignum.c
+++ b/bignum.c
@@ -5283,7 +5283,7 @@ static VALUE
 big_shift(VALUE x, long n)
 {
     if (n < 0)
-	return big_lshift(x, (unsigned long)-n);
+	return big_lshift(x, 1+(unsigned long)(-(n+1)));
     else if (n > 0)
 	return big_rshift(x, (unsigned long)n);
     return x;