PURPOSE: To prevent a sink mark by conducting injection molding through a sandwich expansion molding method by using a thermoplastic synthetic resin material and expanding the thermoplastic synthetic resin material in a continuously connected section, which is thickened, when a reflector, to which a structure is continuously connected integrally and the continuously connected section of which is thickened, is injection-molded by a synthetic resin material.

CONSTITUTION: A mold having a cavity having the same contour shape as a reflector (a semi-finished product) 2 is constituted, the reflector 2 is injection- molded through a sandwich expansion molding method by using a thermoplastic synthetic resin material, and a pivot receiving boss 2d and a screw receiver 2e are continuously connected integrally and molded. A dense hard skin layer 3 is formed along the internal surface of the cavity of the mold, and a core layer 4 is solidified, but an expanded section 6 is formed near a thick section and expansion is displayed at that time. Internal pressure is increased by the expansion, and the skin layer 3 pushed against the mold. Accordingly, there is no possibility, in which a sink mark is generated in the internal surface.