diff --git a/ChangeLog b/ChangeLog
index 864540cdcf..0f6cdd5acd 100644
--- a/ChangeLog
+++ b/ChangeLog
@@ -1,3 +1,7 @@
+Sat Aug  3 00:52:43 2013  Tanaka Akira  <akr@fsij.org>
+
+	* bignum.c (rb_big2str1): Simplify power_level calculation.
+
 Sat Aug  3 00:34:20 2013  Masaki Matsushita  <glass.saga@gmail.com>
 
 	* array.c (rb_ary_zip): use rb_ary_new2() to create buffer
diff --git a/bignum.c b/bignum.c
index ded2d6b272..faedade1e4 100644
--- a/bignum.c
+++ b/bignum.c
@@ -4308,7 +4308,9 @@ big2str_karatsuba(struct big2str_struct *b2s, VALUE x,
 
     /*
      * Precondition:
-     * abs(x) < maxpow_in_bdigit_dbl(base, &numdigits)**(2**power_level)
+     * abs(x) < maxpow**(2**power_level)
+     * where
+     *   maxpow = maxpow_in_bdigit_dbl(base, &numdigits)
      *
      * This function generates sequence of zeros, and then stringized abs(x) into b2s->ptr.
      *
@@ -4453,23 +4455,21 @@ rb_big2str1(VALUE x, int base)
         power = power_cache_get_power(base, power_level, NULL);
     }
     assert(power_level != MAX_BIG2STR_TABLE_ENTRIES);
-    if (FIX2LONG(bary_cmp(BDIGITS(x), RBIGNUM_LEN(x), BDIGITS(power), RBIGNUM_LEN(power))) >= 0) {
+
+    if (RBIGNUM_LEN(power) <= RBIGNUM_LEN(x)) {
+        /*
+         * This increment guarantees x < power_cache_get_power(base, power_level)
+         * without invoking it actually.
+         * (power_cache_get_power(base, power_level) can be slow and not used
+         * in big2str_karatsuba.)
+         *
+         * Although it is possible that x < power_cache_get_power(base, power_level-1),
+         * it is no problem because big2str_karatsuba checks it and
+         * doesn't affect the result when b2s_data.ptr is NULL.
+         */
         power_level++;
     }
 
-#ifndef NDEBUG
-    if (0 < power_level) {
-        VALUE power1 = power_cache_get_power(base, power_level-1, NULL);
-        assert(FIX2LONG(bary_cmp(BDIGITS(x), RBIGNUM_LEN(x), BDIGITS(power1), RBIGNUM_LEN(power1))) >= 0);
-        /*
-        {
-            VALUE power0 = power_cache_get_power(base, power_level, NULL);
-            assert(FIX2LONG(bary_cmp(BDIGITS(x), RBIGNUM_LEN(x), BDIGITS(power0), RBIGNUM_LEN(power0))) < 0);
-        }
-        */
-    }
-#endif
-
     b2s_data.negative = RBIGNUM_NEGATIVE_P(x);
     b2s_data.base = base;
     b2s_data.hbase = maxpow_in_bdigit(base, &b2s_data.hbase_numdigits);