PURPOSE: To stabilize welding and to increase welding speed by bending the end of a weld zone, opening the end faces and melting the opened end face parts thereby welding the same.

CONSTITUTION: Thin metallic sheets 11 are bent at prescribed intervals to form many heat radiating ribs 1. The end 3 of each rib 1 is crushed by clamping dies 23. A core die 24 is inserted therein to form a flow passage 22 for oil and an end face die 25 is inserted therein from the transverse direction, then the end face 31 of the rib 1 is crushed to a Y-shape. The die 25 is drawn out and an arc is started at the top end 6 of the rib 1 by a torch 7. The torch 7 is progressed downward after stabilization of the arc to execute welding 4 of the end face 31. The molten metal is held without flowing down by the balance between the weight of the molten metal and the surface tension in the above-mentioned method, by which the welding is stably executed. The welding speed is therefore increased.