PURPOSE: To exhaust fully the inner part of a cavity to mold a good molded piece and to prevent flashes from occurring by a method wherein an inner pin for molding the inner surface of a molded piece is supported by a specified means so that a communication port (exhaust port) to a cavity can be opened or closed by the inner pin.

CONSTITUTION: When a molded piece with a cavity is molded, the inner pin 11e to mold the cavity is supported by the pressure of a spring, a rubbery elastic material, an air cylinder or an oil cylinder. Before plasticized material (molding material) is injected into the cavity 18, this inner pin 11e is pushed into the cavity 18 to make the communication ports 16, 36 open state so that sufficient air discharge can be conducted. When the molding material is injected into the cavity 18 after air discharge, the inner pin 11e is returned and the communication ports 16, 36 are closed by the inner pin 11e to prevent flashes from being formed.