PURPOSE: To simplify combination work and to prevent the generation of bending or distortion, by inserting a core material obtained in a separate process in a molding main body in such a state that said main body is yet held to a high temp. state before contracting the molding main body by natural cooling.

CONSTITUTION: A molding main body 10 and a core material 20 respectively molded in separate processes are mutually combined by inserting the core material 20 in a space 11 in such a state that the molding main body 10 is yet held to high temp. state and, when both of them allowed to stand after combination so as to be cooled naturally, the molding main body 10 gradually generates volumetric contraction as a whole to strongly tighten the core material 20. In addition thereto, a heat activatable adhesive 24 is preliminarily adhered to the surface opposed to the inner surface of the space of the core material 20 and, after this core material 20 was inserted in the molding main body 10, the whole is inserted in a mold and subjected to heat treatment by a high frequency heater 40 to heat the metal core material and the adhesive 24 is activated to make it possible to further strongly fix both of them to each other.