diff --git a/sprintf.c b/sprintf.c
index 70c7cceb14d6..db7499979dc3 100644
--- a/sprintf.c
+++ b/sprintf.c
@@ -1135,6 +1135,8 @@ rb_str_format(int argc, const VALUE *argv, VALUE fmt)
 		fval = RFLOAT_VALUE(rb_Float(val));
 		if (isnan(fval) || isinf(fval)) {
 		    const char *expr;
+		    int elen;
+		    char sign = '\0';
 
 		    if (isnan(fval)) {
 			expr = "NaN";
@@ -1143,33 +1145,28 @@ rb_str_format(int argc, const VALUE *argv, VALUE fmt)
 			expr = "Inf";
 		    }
 		    need = (int)strlen(expr);
-		    if ((!isnan(fval) && fval < 0.0) || (flags & FPLUS))
-			need++;
+		    elen = need;
+		    i = 0;
+		    if (!isnan(fval) && fval < 0.0)
+			sign = '-';
+		    else if (flags & (FPLUS|FSPACE))
+			sign = (flags & FPLUS) ? '+' : ' ';
+		    if (sign)
+			++need;
 		    if ((flags & FWIDTH) && need < width)
 			need = width;
 
-		    CHECK(need + 1);
-		    snprintf(&buf[blen], need + 1, "%*s", need, "");
+		    FILL(' ', need);
 		    if (flags & FMINUS) {
-			if (!isnan(fval) && fval < 0.0)
-			    buf[blen++] = '-';
-			else if (flags & FPLUS)
-			    buf[blen++] = '+';
-			else if (flags & FSPACE)
-			    blen++;
-			memcpy(&buf[blen], expr, strlen(expr));
+			if (sign)
+			    buf[blen - need--] = sign;
+			memcpy(&buf[blen - need], expr, elen);
 		    }
 		    else {
-			if (!isnan(fval) && fval < 0.0)
-			    buf[blen + need - strlen(expr) - 1] = '-';
-			else if (flags & FPLUS)
-			    buf[blen + need - strlen(expr) - 1] = '+';
-			else if ((flags & FSPACE) && need > width)
-			    blen++;
-			memcpy(&buf[blen + need - strlen(expr)], expr,
-			       strlen(expr));
+			if (sign)
+			    buf[blen - elen - 1] = sign;
+			memcpy(&buf[blen - elen], expr, elen);
 		    }
-		    blen += strlen(&buf[blen]);
 		    break;
 		}
 
diff --git a/test/ruby/test_sprintf.rb b/test/ruby/test_sprintf.rb
index 3fd4736a54b0..ab3037a8f267 100644
--- a/test/ruby/test_sprintf.rb
+++ b/test/ruby/test_sprintf.rb
@@ -83,6 +83,18 @@ def test_nan
     assert_equal("NaN", sprintf("%-f", nan))
     assert_equal("+NaN", sprintf("%+f", nan))
 
+    assert_equal("NaN", sprintf("%3f", nan))
+    assert_equal("NaN", sprintf("%-3f", nan))
+    assert_equal("+NaN", sprintf("%+3f", nan))
+
+    assert_equal(" NaN", sprintf("% 3f", nan))
+    assert_equal(" NaN", sprintf("%- 3f", nan))
+    assert_equal("+NaN", sprintf("%+ 3f", nan))
+
+    assert_equal(" NaN", sprintf("% 03f", nan))
+    assert_equal(" NaN", sprintf("%- 03f", nan))
+    assert_equal("+NaN", sprintf("%+ 03f", nan))
+
     assert_equal("     NaN", sprintf("%8f", nan))
     assert_equal("NaN     ", sprintf("%-8f", nan))
     assert_equal("    +NaN", sprintf("%+8f", nan))
@@ -106,6 +118,26 @@ def test_inf
     assert_equal("Inf", sprintf("%-f", inf))
     assert_equal("+Inf", sprintf("%+f", inf))
 
+    assert_equal(" Inf", sprintf("% f", inf))
+    assert_equal(" Inf", sprintf("%- f", inf))
+    assert_equal("+Inf", sprintf("%+ f", inf))
+
+    assert_equal(" Inf", sprintf("% 0f", inf))
+    assert_equal(" Inf", sprintf("%- 0f", inf))
+    assert_equal("+Inf", sprintf("%+ 0f", inf))
+
+    assert_equal("Inf", sprintf("%3f", inf))
+    assert_equal("Inf", sprintf("%-3f", inf))
+    assert_equal("+Inf", sprintf("%+3f", inf))
+
+    assert_equal(" Inf", sprintf("% 3f", inf))
+    assert_equal(" Inf", sprintf("%- 3f", inf))
+    assert_equal("+Inf", sprintf("%+ 3f", inf))
+
+    assert_equal(" Inf", sprintf("% 03f", inf))
+    assert_equal(" Inf", sprintf("%- 03f", inf))
+    assert_equal("+Inf", sprintf("%+ 03f", inf))
+
     assert_equal("     Inf", sprintf("%8f", inf))
     assert_equal("Inf     ", sprintf("%-8f", inf))
     assert_equal("    +Inf", sprintf("%+8f", inf))
@@ -126,6 +158,26 @@ def test_inf
     assert_equal("-Inf", sprintf("%-f", -inf))
     assert_equal("-Inf", sprintf("%+f", -inf))
 
+    assert_equal("-Inf", sprintf("% f", -inf))
+    assert_equal("-Inf", sprintf("%- f", -inf))
+    assert_equal("-Inf", sprintf("%+ f", -inf))
+
+    assert_equal("-Inf", sprintf("% 0f", -inf))
+    assert_equal("-Inf", sprintf("%- 0f", -inf))
+    assert_equal("-Inf", sprintf("%+ 0f", -inf))
+
+    assert_equal("-Inf", sprintf("%4f", -inf))
+    assert_equal("-Inf", sprintf("%-4f", -inf))
+    assert_equal("-Inf", sprintf("%+4f", -inf))
+
+    assert_equal("-Inf", sprintf("% 4f", -inf))
+    assert_equal("-Inf", sprintf("%- 4f", -inf))
+    assert_equal("-Inf", sprintf("%+ 4f", -inf))
+
+    assert_equal("-Inf", sprintf("% 04f", -inf))
+    assert_equal("-Inf", sprintf("%- 04f", -inf))
+    assert_equal("-Inf", sprintf("%+ 04f", -inf))
+
     assert_equal("    -Inf", sprintf("%8f", -inf))
     assert_equal("-Inf    ", sprintf("%-8f", -inf))
     assert_equal("    -Inf", sprintf("%+8f", -inf))