In the present invention, while a welding current is applied with respect to a laminate body comprising thick metal plates and thin metal plates, a first welding pressure (F1) with respect to the laminate body from an upper tip contacting the metal plate and a second welding pressure (F2) with respect to the laminate body from a lower tip contacting the metal plate are changed relative to each other. Specifically, F1 F2 in a third step which is a final period of welding.