diff --git a/ChangeLog b/ChangeLog
index 5797e3bf8c..2b266ba16c 100644
--- a/ChangeLog
+++ b/ChangeLog
@@ -1,3 +1,9 @@
+Mon Feb 25 15:33:29 2008  NAKAMURA Usaku  <usa@ruby-lang.org>
+
+	* bignum.c (big2str_karatsuba): initialize cache if not initialized.
+
+	* bignum.c (Init_Bignum): delayed initializing cache.
+
 Mon Feb 25 13:40:03 2008  Tanaka Akira  <akr@fsij.org>
 
 	* process.c (Init_process): share bignum objects for RLIM_INFINITY,
diff --git a/bignum.c b/bignum.c
index 7220b80a36..773cba30ba 100644
--- a/bignum.c
+++ b/bignum.c
@@ -737,6 +737,7 @@ ceil_log2(register unsigned long x)
 #define MAX_BIG2STR_TABLE_ENTRIES 64
 
 static VALUE big2str_power_cache[35][MAX_BIG2STR_TABLE_ENTRIES];
+static int power_cache_initialized = 0;
 
 static void
 power_cache_init(void)
@@ -901,6 +902,10 @@ big2str_karatsuba(VALUE x, int base, char* ptr,
         return big2str_orig(x, base, ptr, len, hbase, trim);
     }
 
+    if (!power_cache_initialized) {
+	power_cache_init();
+	power_cache_initialized = 1;
+    }
     b = power_cache_get_power(base, n1, &m1);
     bigdivmod(x, b, &q, &r);
     lh = big2str_karatsuba(q, base, ptr,      (len - m1)/2,
@@ -2605,6 +2610,4 @@ Init_Bignum(void)
     rb_define_method(rb_cBignum, "size", rb_big_size, 0);
     rb_define_method(rb_cBignum, "odd?", rb_big_odd_p, 0);
     rb_define_method(rb_cBignum, "even?", rb_big_even_p, 0);
-
-    power_cache_init();
 }