Abstract: A decoration sheet is retained on the cavity periphery by a sheet clamp in such a state that it is projected into a molding cavity of a female die. Then the decoration sheet is heated and softened by a curved hot platen, and thereafter it is drawn through suction ports of the female die to bring the sheet into close fit to the inner periphery of the molding cavity. The deformation amount of the decoration sheet heated and softened is decreased in premolding the sheet into the molding cavity surface of the female die. Therefore, distortion of the decoration sheet, appearance of wrinkles, and breakage thereof become little. Next, the female die and the male die are coupled with each other to effect die clamping, and a molten resin is poured to fill the molding cavity to effect injection molding.