diff --git a/ChangeLog b/ChangeLog
index aa86bec134..f9fa1e7af2 100644
--- a/ChangeLog
+++ b/ChangeLog
@@ -1,3 +1,8 @@
+Sat Jun  1 00:19:50 2013  Tanaka Akira  <akr@fsij.org>
+
+	* bignum.c (calc_hbase): Make hbase the maximum power of base
+	  representable in BDIGIT.
+
 Fri May 31 23:56:13 2013  Tanaka Akira  <akr@fsij.org>
 
 	* bignum.c (calc_hbase): Extracted from rb_big2str0.
diff --git a/bignum.c b/bignum.c
index a19560a609..b8ff3fe76e 100644
--- a/bignum.c
+++ b/bignum.c
@@ -1125,12 +1125,12 @@ calc_hbase(int base, long *hbase_p, int *hbase_numdigits_p)
     long hbase;
     int hbase_numdigits;
 
-    hbase = base*base;
-    hbase_numdigits = 2;
-#if SIZEOF_BDIGITS > 2
-    hbase *= hbase;
-    hbase_numdigits *= 2;
-#endif
+    hbase = base;
+    hbase_numdigits = 1;
+    while (hbase <= (~(BDIGIT)0) / base) {
+        hbase *= base;
+        hbase_numdigits++;
+    }
 
     *hbase_p = hbase;
     *hbase_numdigits_p = hbase_numdigits;