A molding die is devised to suppress the formation of a fragile layer, thereby ensuring stable production of a resin molded body having remarkably improved adhesion of a plating film. A plurality of stepped parts are formed so as to be spaced apart from each other on a second die surface which molds a back surface on the side opposite to a design surface to which metal plating is to be applied, along the main flowing direction of a molten resin and in a direction crossing the main flowing direction. Since the flow of the molten resin changes due to the stepped parts and the influence thereof extends even to the vicinity of the design surface, the formation of a fragile layer is suppressed.