diff --git a/ChangeLog b/ChangeLog
index 7d2f51d55e..3bb1a63e42 100644
--- a/ChangeLog
+++ b/ChangeLog
@@ -1,3 +1,7 @@
+Sat Mar  1 02:35:08 2008  Nobuyoshi Nakada  <nobu@ruby-lang.org>
+
+	* bignum.c (big2str_find_n1): check integer overflow.
+
 Sat Mar  1 00:29:07 2008  Nobuyoshi Nakada  <nobu@ruby-lang.org>
 
 	* encoding.c (rb_enc_dummy_p): bootstrap encodings can not be dummy.
diff --git a/bignum.c b/bignum.c
index 773cba30ba..649b258c53 100644
--- a/bignum.c
+++ b/bignum.c
@@ -829,6 +829,9 @@ big2str_find_n1(VALUE x, int base)
     else if (BIGZEROP(x)) {
         return 0;
     }
+    else if (RBIGNUM_LEN(x) >= LONG_MAX/BITSPERDIG) {
+	rb_raise(rb_eRangeError, "bignum too big to convert into `string'");
+    }
     else {
         bits = BITSPERDIG*RBIGNUM_LEN(x);
     }
diff --git a/test/ruby/test_bignum.rb b/test/ruby/test_bignum.rb
index 2d9008ecce..be33e2f5af 100644
--- a/test/ruby/test_bignum.rb
+++ b/test/ruby/test_bignum.rb
@@ -377,4 +377,13 @@ class TestBignum < Test::Unit::TestCase
   def test_interrupt
     assert(interrupt { (65536 ** 65536).to_s })
   end
+
+  def test_too_big_to_s
+    i = 32
+    while (big = 2**(i-1)-1).is_a?(Fixnum)
+      i *= 2
+    end
+    e = assert_raise(RangeError) {(1 << big).to_s}
+    assert_match(/too big to convert/, e.message)
+  end
 end