diff --git a/ChangeLog b/ChangeLog
index 0ce0521314..2adbd80de8 100644
--- a/ChangeLog
+++ b/ChangeLog
@@ -1,3 +1,7 @@
+Wed Jul 13 14:28:33 2016  NARUSE, Yui  <naruse@ruby-lang.org>
+
+	* math.c (rb_math_sqrt): r55646 must use f_signbit.
+
 Wed Jul 13 14:22:50 2016  Koichi Sasada  <ko1@atdot.net>
 
 	* iseq.c (Init_ISeq): undef ISeq.translate and ISeq.load_iseq
diff --git a/math.c b/math.c
index 8d36389a28..9cdf9e5565 100644
--- a/math.c
+++ b/math.c
@@ -591,13 +591,31 @@ math_sqrt(VALUE obj, VALUE x)
     return rb_math_sqrt(x);
 }
 
+#define f_boolcast(x) ((x) ? Qtrue : Qfalse)
+inline static VALUE
+f_negative_p(VALUE x)
+{
+    if (FIXNUM_P(x))
+        return f_boolcast(FIX2LONG(x) < 0);
+    return rb_funcall(x, '<', 1, INT2FIX(0));
+}
+inline static VALUE
+f_signbit(VALUE x)
+{
+    if (RB_TYPE_P(x, T_FLOAT)) {
+        double f = RFLOAT_VALUE(x);
+        return f_boolcast(!isnan(f) && signbit(f));
+    }
+    return f_negative_p(x);
+}
+
 VALUE
 rb_math_sqrt(VALUE x)
 {
     double d;
 
     if (RB_TYPE_P(x, T_COMPLEX)) {
-	int neg = signbit(RCOMPLEX(x)->imag);
+	int neg = f_signbit(RCOMPLEX(x)->imag);
 	double re = Get_Double(RCOMPLEX(x)->real), im;
 	d = Get_Double(rb_complex_abs(x));
 	im = sqrt((d - re) / 2.0);