PURPOSE: To thin a thin section extremely without being affected by the viscosity of a molten resin by forming the thin section of an injection molded piece by the pushing force of a molding pin during the filling of the resin and shaping a thick section by injection pressure.

CONSTITUTION: A molten resin is filled continuously into a cavity 5 through a gate 4 from a sprue runner, the tip section of a molding pin 14 is brought near to the surface of the cavity 5 gradually when part of the molten resin M crosses over the tip section of the molding pin 14 and begins to flow through a cavity extension section 7 through a communicating path 6, the molten resin is filled continuously even during the forward movement of the molding pin, and the molding pin is stopped when a clearance (w) between the tip section of the molding pin and the surface 5a of the cavity reaches a set value. The resin is flowed through the communicating path 6 and the cavity extension section 7 is filled with the resin. The resin is cooled, the movable side mold of a movable side template 2, a backing plate 3, etc. is retreated, and an injection molded piece is mold-released.