PURPOSE: To prevent an upper and a lower electrode from wearing and to improve the reliability by preheating two members to be welded by frictional heat generated by sliding those two members sandwiched between the upper and lower electrodes and then welding those members, and thus performing the resistance welding of both the members without welding the members to the electrodes.

CONSTITUTION: Weld zones of members 6 and 6' to be welded are set nearly in the centers of electrodes 1 and 1'. Then, a pressing mechanism member 3 is operated to move the upper electrode 1' downward, thereby clamping the members 6 and 6' to be welded. Then, a preliminary power source 7 flows currents small enough not to advance the fusion of the members 6 and 6' to the electrodes 1 and 1'. At the same time, a slide mechanism member 81 is operated to move the upper electrode 1' and member 6' forth and back synchronously, preheating the weld zones. Then, the operation of the slide mechanism member 81 is stopped and a welding power source 5 flows a welding current whose voltage and welding time are controlled to perform the resistance welding of the members 6 and 6'.