PURPOSE: To easily mold undercut having excellent strength and appearance even when a material having high rigidity is used, by constituting an outer mold and a core mold of a mold for a resin molded piece of an incompressible elastic body such as rubber and elastomer.

CONSTITUTION: A mold is constituted of a first outer mold 1, a second outer mold 2 and a core mold 3 and a thermoplastic resin material (which has rigidity after solidification) of about 200-300°C is injected from a gate G to a cavity C. The core mold 3 is fixed to a support member 4 which is slidable to the second outer mold 2, and is integrally fixed to the second outer mold 2 at injection molding. The core mold 3 is formed into almost a cylindrical shape which is widened at its tip in accordance with a shape of a molded piece by an incompressible material such as vulcanized rubber and elastomer so as to form undercut on the molded piece. The core mold 3 is set in the second outer mold 2 and the mold is closed together with the first outer mold 1 so as to inject the thermoplastic resin material into a formed cavity C.