PURPOSE: To prolong the life of electrode tips by pressing the pressurizing surface of a back bar to a plate to be welded and pressing the contact surface on the side opposite from this contact surface to the one tip at the time of crimping the plates to be welded.

CONSTITUTION: The plates 26 which are to be welded and are superposed between a pair of the tips 30 and 43 are crimped under pressurization and a large current is passed thereto in a short period of time to weld the local part thereof. The back bar 35 is, thereupon, supported via a floating means to a frame 28 mounted with the one tip 43 on the front side of the tip 43. The back bar 35 is so set that the pressurizing surface of the back bar is pressed to the plate 26 to be welded and that the contact surface on the side opposite from this contact surface is pressed to the one tip 43 when the back bar is moved by overcoming the energizing force of the floating means. The operation to exchange the back bar in correspondence to the plates to be welded is easily carried out in this way.