PURPOSE: To obtain uniform and excellent seam welding effect by lapping painted steel plates and starting seam welding after inserting a conductive shunt bar to the starting end of welding and thereby destructing the paint film of the starting point of welding completely.

CONSTITUTION: Painted steel plates 2, 3 are lapped, and a nearly U-shaped shunt bar 1 made of a conductive material such as Cu etc. is inserted from the starting end of welding to the direction of welding. At this time, the shunt bar 1 is made the starting point of welding and seam welding electrode wheels 4a, 4b are brought into contact. Then, applied welding current flows through the shunt bar 1, and the paint film just below it is destructed by heat generated at this time, and this part is welded completely. This initial heat generation destructs the paint film along the weld line successively, and consequently, succeeding seam welding is performed completely. After completion of welding, bent part of the shunt bar 1 is cut by fusion. As the lower side falls down automatically, the upper part can be removed easily.