PURPOSE: To prevent occurrence of casting inferiority in the periphery of a core, by forming a hollow part in the inside of core and installing a communicating hole between the hollow part and the inside of a mold and allowing a molten metal to flow into the hollow part of core through the communicating hole.

CONSTITUTION: When a molten metal is poured from a gate system 3 into a mold, the molten metal flows into the mold and pouring is performed completely. The molten metal, at the same time, flows into the hollow part 6 of a core 5 and the hollow part 6 is filled with the molten metal. After pouring, the molten metal solidifies gradually, and the cooling of thickened part of a cylindrical part 4b is made gradually by the core containing the molten metal in its hollow part 6. Moreover, since the temperature of the cylindrical part 4b is lowered by maintaining a balance between its outside and inside, a good casting is produced without generating any internal porosity, etc., at the thickened part of cylinder 4b. The gas thus produced flows from the inside of mold into the hollow part 6 of the core 5 and is enclosed in the hollow part 6, and thus occurrence of defects in casting products by gas is prevented.