In mash seam welding for performing to weld metal plates with thicknesses of 2 mm or more, it is possible to stabilize the strength of a joint portion, reduce uneven parts of the joint portion, ensure high strength and reliability of the joint portion, and join metal plates with thicknesses of 4.5 mm or more. Two metal plates (5, 6) are clamped with first and second clamping devices (7, 8), and the two metal plates (5, 6) are placed to overlap each other at their end portions, and then overlapping portions (L) of the two metal plates 5, 6) are pressed with a pair of upper and lower electrode wheels (1, 2) while welding currents flow to continuously weld the overlapping portions (L) and join the two metal plates. The pessing forces applied to the upper and lower electrode wheels (1, 2) 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 while the welding currents flow.