PURPOSE: To uniformly penetrate an arc by rotating it by electro-magnetic force and to obtain the weld zone of high quality by performing MIG or TIG arc spot welding with passing the alternating exciting current of single phase or three phase to the exciting coil provided at the torch tip.

CONSTITUTION: A six pole iron core 21 and exciting coil 22 are arranged on the periphery of the arc generating part of the welding torch 4 tip. The welding is then performed with rotating the arc by the electro-magnetic force by passing three phase exciting current from an exciting power source 23 to exciting coil 22. Consequently, no arc is concentrated in one place, a uniform penetration shape is obtd., and the thermal stress is reduced, and high temp. crack generation can be prevented.