PURPOSE: To prevent the generation of the scorch and sink of a resin by injecting gas into a cavity by a gas injection device when or after a molten resin is injected.

CONSTITUTION: The decorated film X heated along with a hot plate 9 is brought into close contact with the cavity constituting surface 2a of a female mold 2 and the female mold 2 and a male mold 1 are fitted by the operation of a ram 7 and a molten resin is discharged to the cavity between both molds 1, 2 in a mold clamping state to be molded. Inert gas (e.g. N2) is injected in the cavity during the discharge of the molten resin from the gas nozzle GN of a gas injection device G, an injection nozzle 5 and an injection gate 4. The inert gas is injected into the molten resin 40 to form a space 43 and the molten resin 40 is extruded to the outside by the pressure of the inert gas. Therefore, the molten resin is solidified when it is allowed to stand for a predetermined time and a molded product is completed but the transfer property of the transfer terminal of the molded product is enhanced by the pressure of the inert gas and no sink is generated on the surface of the molded product.