A part has separator layers of resin laminated on each other with heat-resistant layers interposed among them and a sheath (laminated sheet) laid on each side of the laminated separator layers. The part is held, pressurized, and vibrated by a pressure vibrator and a jig receiver. The pressure vibrator and jig receiver are provided with projections that pressurize, vibrate, and break the heat-resistant layers, thereby melting and welding together the resin of the separator layers at the broken part.