To reduce strain, warping, twisting, etc., in a casting.

In the casting having outer walls 7, 7, 9, 9 formed with a thinning structure of the thickness and rib structure produced with casting formed with the ribs (the first ribs 5, 5, the second ribs 6, 6, the third ribs 8, 8) at the inner side of the outer walls (side walls) 7, 7, 9, 9, each thickness of the outer walls (side walls) 7, 7, etc., are thicker than each thickness of the ribs (the first ribs 5, 5, etc.).