Mash seam welding is performed to weld metal plates with thicknesses of 2 mm or more to stabilize the strength of a joint portion, reduce uneven parts of the joint portion, and ensure high strength and reliability of the joint portion. First, two metal plates, clamped with first and second clamping devices, are placed to overlap each other at their end portions. The overlapping portions (L) of the metal plates are pressed together with a pair of upper and lower electrode wheels while welding currents flow to continuously weld the overlapping portions (L) and join the metal plates. The pressing forces applied to the electrode wheels are controlled so that upper and lower welding forces acting upon the overlapping portions are equal to each other during the time when the overlapping portions are continuously welded.