PURPOSE: To prevent the penetration of metal to the recessed part of a sand mold and to obtain a good casting surface by forming at least a part of cavity with the sand mold and arranging a gap part having a specific value or lower successively connected with the product cavity.

CONSTITUTION: The gap 6 having ≤1mm thickness is arranged on a parting surface 5 of a mold 1 successively connected with the cavity 3 in the mold 1 made of the sand mold. The penetration developed by flowing molten metal into fine ruggedness on the sand mold surface, is developed when the dynamic pressure changes into the static pressure by stop of the flowing of the molten metal. The molten metal is flowed into the gap 6 by arranging the gap 6, and action force to the recessed surface of the mold 1 is weakened by the static pressure to prevent the penetration of metal. Since the thin piece can be developed at the position of the gap 6 in the casting just after casting into the mold 1, but can easily be separated at the time of knocking out the mold, this removing work is unnecessary.