To increase the strength of a thick-walled part and the fatigue strength, and to improve the machinability by setting a modulus-designed chiller in a thick-walled part casting cavity as a core, and cooling the thick-walled part of the casting to refine the graphite grain size of the thick-walled part similar to the graphite grain size of a thin-walled part.

A core to be set in a thick-walled casting cavity having a through hole is a chiller. The solidifying speed of the molten metal is increased thereby, and the nodular graphite grain size is reduced. In addition, because the passing speed at the eutectoid transformation point is reduced through the heat insulation effect by the heat of the molten metal stored in the chiller, the ferrite quantity can be secured, and the metallic structure excellent in the mechanical properties can be provided even at the thick-walled part. The mean cooling speed from the temperature immediately above the eutectic solidification starting temperature by 100K to the eutectic solidification temperature is preferably 5-10 K/sec. In addition, when the chiller modulus-designed so that the eutectoid transformation point passing speed is ≤1.0 K/sec., is set, the graphite is refined, and the matrix of the thick-walled part can be ferrite.