PURPOSE: To save a mold cost by reducing a number of mold pieces, and improve productivity, by making an upper mold for molding a core and an upper mold for molding a skin common use and molding them at a single molding stage when molding the interior part composed of the core and the skin.

CONSTITUTION: A core is molded by moving the lower mold 14 for molding the core from a waiting stage b to a molding stage (a), lowering an upper mold 18 and pouring raw material of the core from an injecting head 22 into a cavity shaped between the upper mold 18 and the lower mold 14. Then, the molded core is raised, being held by the upper part 18, the lower mold 14 for molding the core is returned to the original waiting stage b and also the lower mold 15 for molding a skin is moved from the wairint stage c to the molding stage (a). The upper mold 18 holding the core is lowered and an aimed interior part is obtained by pouring raw material of the skin from the injecting head 27 to the cavity shaped between the lower mold 15 and the core and molding the skin.