diff --git a/ChangeLog b/ChangeLog
index 51416fcd25..15ce86f766 100644
--- a/ChangeLog
+++ b/ChangeLog
@@ -1,3 +1,7 @@
+Thu Jan 14 16:12:09 2016  Nobuyoshi Nakada  <nobu@ruby-lang.org>
+
+	* sprintf.c (rb_str_format): format exact number more exactly.
+
 Thu Jan 14 15:08:43 2016  Tony Arcieri  <bascule@gmail.com>
 
 	* Remove 512-bit DH group. It's affected by LogJam Attack.
diff --git a/sprintf.c b/sprintf.c
index 84d5fd112b..1984cec805 100644
--- a/sprintf.c
+++ b/sprintf.c
@@ -1036,13 +1036,19 @@ rb_str_format(int argc, const VALUE *argv, VALUE fmt)
 		int sign = (flags&FPLUS) ? 1 : 0, zero = 0;
 		long len, done = 0;
 		int prefix = 0;
-		if (!RB_TYPE_P(val, T_RATIONAL)) {
+		if (FIXNUM_P(val) || RB_TYPE_P(val, T_BIGNUM)) {
+		    den = INT2FIX(1);
+		    num = val;
+		}
+		else if (RB_TYPE_P(val, T_RATIONAL)) {
+		    den = rb_rational_den(val);
+		    num = rb_rational_num(val);
+		}
+		else {
 		    nextvalue = val;
 		    goto float_value;
 		}
 		if (!(flags&FPREC)) prec = default_float_precision;
-		den = rb_rational_den(val);
-		num = rb_rational_num(val);
 		if (FIXNUM_P(num)) {
 		    if ((SIGNED_VALUE)num < 0) {
 			long n = -FIX2LONG(num);
diff --git a/test/ruby/test_sprintf.rb b/test/ruby/test_sprintf.rb
index 714ef8b4e2..e196e7cfbd 100644
--- a/test/ruby/test_sprintf.rb
+++ b/test/ruby/test_sprintf.rb
@@ -149,6 +149,11 @@ class TestSprintf < Test::Unit::TestCase
     assert_equal(" Inf", sprintf("% e", inf), '[ruby-dev:34002]')
   end
 
+  def test_bignum
+    assert_match(/\A10{120}\.0+\z/, sprintf("%f", 100**60))
+    assert_match(/\A10{180}\.0+\z/, sprintf("%f", 1000**60))
+  end
+
   def test_rational
     assert_match(/\A0\.10+\z/, sprintf("%.60f", 0.1r))
     assert_match(/\A0\.010+\z/, sprintf("%.60f", 0.01r))