PURPOSE: To control the generation of sink marks on the surface of a product by installing a pressurizing means for pushing out a product into a visible face side on the mold side in contact with an invisible face of the product and setting the bonding force of invisible face side of the product smaller than that of the visible face side by the pressurizing means.

CONSTITUTION: A core 2 on the invisible side of a product and a cavity 3 on the visible side of the product are set to be opened and closed, and a recessed section 4 corresponding to the external shape of an injection molded product (a) is formed on the core 2, and molten plastic is poured from an injection molding machine into the recessed section 4 to form the plastic molded product (a) through a sprue 5 on the given site of the cavity 3. A fluid such as compressed air is injected from outside into between an invisible face (d) of the molded product and the core 2 through a ventilation tube 7 and a sintered metal material 6. As a result, the control of the positions for the generation of sink marks can be performed so that the bonding force of resin of the invisible face (d) of the molded product and the surface of the core 2 gets smaller to eliminate the generation of sink marks on a visible face (e) of the plastic molded product and to generate sink marks on the invisible face (d) of the molded product.