diff --git a/ChangeLog b/ChangeLog
index 33848903a7..00d1a3a14f 100644
--- a/ChangeLog
+++ b/ChangeLog
@@ -1,3 +1,9 @@
+Sun Aug 30 09:45:11 2009  Tanaka Akira  <akr@fsij.org>
+
+	* bignum.c (bigmul1_single): new function specialized respect to
+	  multiply two single digit bignums.
+	  (bigmul0): use bigmul1_single.
+
 Sun Aug 30 03:59:43 2009  Tanaka Akira  <akr@fsij.org>
 
 	* timev.h (TIME_SCALE): defined as 1000000000.
diff --git a/bignum.c b/bignum.c
index 72a250e31f..ca48dea882 100644
--- a/bignum.c
+++ b/bignum.c
@@ -1745,6 +1745,24 @@ big_real_len(VALUE x)
     return i + 1;
 }
 
+static VALUE
+bigmul1_single(VALUE x, VALUE y)
+{
+    BDIGIT_DBL n;
+    VALUE z = bignew(2, RBIGNUM_SIGN(x)==RBIGNUM_SIGN(y));
+    BDIGIT *xds, *yds, *zds;
+
+    xds = BDIGITS(x);
+    yds = BDIGITS(y);
+    zds = BDIGITS(z);
+
+    n = (BDIGIT_DBL)xds[0] * yds[0];
+    zds[0] = BIGLO(n);
+    zds[1] = BIGDN(n);
+
+    return z;
+}
+
 static VALUE
 bigmul1_normal(VALUE x, VALUE y)
 {
@@ -2022,6 +2040,7 @@ bigmul0(VALUE x, VALUE y)
     if (xn < KARATSUBA_MUL_DIGITS) {
       normal:
 	if (x == y) return bigsqr_fast(x);
+	if (xn == 1 && yn == 1) return bigmul1_single(x, y);
     	return bigmul1_normal(x, y);
     }