A semiconductor device manufacturing method allows stably forming a plating layer at low cost on one main surface side of a substrate, while preventing unintended plating layer deposition on the other main surface side. Emitter and collector electrodes are respectively formed on the front and back surfaces of a semiconductor substrate. A first film is attached to the back surface. A notch portion of the substrate is filled with a resin member. A second film is attached to an outer peripheral portion of the substrate, straddling the substrate from the front surface to the back surface. The first and second films push out air remaining between the first and second films and the substrate. An electroless plating process is carried out while the first and second films are attached to the substrate, thereby sequentially forming a nickel plating layer and a gold plating layer on the front surface side.