PURPOSE: To prevent the air caught in resin during molding and molding failure from developing by a method wherein a flexible and expansible film is interposed between molds so as to inject resin in a cavity, ofter the air in the cavity is exhausted by bringing the film in close contact with the cavity of one mold by pressure, in order to fill the cavity with the resin.

CONSTITUTION: A flexible and expansible film 4, which is made of urethane rubber, silicone rubber, polyester rubber or the like, is interposed between the mold parting faces of a top force 2 and a bottom force 3. By supplying high pressure air to the top force 2 after the mold closure by lowering the top force 2, the jetted air pressure brings the film 4 in close contact with the inner surface of a recessed part 304, resulting in exhausting the air in a cavity 5. Next, liquid resin is injected through a gun 306 from a nozzle 307 in the cavity 5. Since the air in the cavity is exhausted completely, the resin to fill the cavity does not contact the air, resulting in preventing molding failure due to the air caught in the resin and the resin in contact with the air from developing.