PURPOSE: To reduce sharply the number of defective parts due to trapped air, by allowing the air dwelling in a mold when the injection molding is carried out to escape, with time of the injection of the molding raw material, through an air introducing channel and an air retaining recessed section from an air escaping path so that the air may be discharged outside of the mold.

CONSTITUTION: A one end of an air escaping path 11 in a bottom force 2 is open in an upper surface of a projected section 4 of the bottom force 2, the projected section 4 serving for forming a through-hole in the intended item. The upper surface of the projected section 4 is the mating surface 4a that will be mated with a recess 5 of a top force 1. The other end of the path 11 is open to the outside of the mold, and the mating surface 4a is formed with an air retaining recess 12 located adjacent to the opening of the air escaping path 11. Further the mating surface 4a is provided with an air introducing channel 13 to allow the section adjacent to the projected section 4 where air will dwell to be in communication with the air retaining recess 12. When a raw material is injected into the closed mold, a gas will stay near the air introducing channel 13, then dwell in the retaining recess 12 and finally is allowed to be discharged via the channel 11.