PURPOSE: To mold the reflecting surface in which the strain due to the residual stress in a film does not occur by a method in which a thermoplastic resin film is sucked and heated by a platen, and is transferred to the front surface of a movable mold, and a reflecting plate is injection-molded from the state where the heated resin film is brought in contact with the cavity-constituting surface of the movable and is stuck by suction thereto.

CONSTITUTION: In the injection-molding of an antenna body B, the body B is simultaneously covered with resin films X, thereby forming a current-reflecting layer X2 and a weathering resistant layer X3. Since the vacuum and pressurizing molding of the resin film X is achieved by sufficiently softening the resin film X by a platen, even if the resin film X is sucked on a cavity-constituting surface 39 by vacuum and pressurizing molding and is deformed with its extension so that the film X is brought in contact with the cavity-constituting surface 39 by injection molding, the stress accompanied by its elongation and deformation does not occur in the film. Consequently, in the produced reflecting plate A, residual stress does not exist in the resin film with which the reflecting surface is covered, whereby the strain due to the residual stress does not occur on the reflecting surface.