PURPOSE: To stabilize opposite polarity arc and improve welding efficiency by forming oxide film on the surface of filler metal and supplying the same to molten pool.

CONSTITUTION: In performing opposite polarity welding by a welding torch 1, filler metal 3 is fed into molten pool 5 at the specified speed by plural feed rollers, etc. At this time, the filler metal 3 is perheated by using a TIG welding torch 7 (an electric heater is equally well) and an oxide film 32 is formed on the surface of the filler metal 3 by the O2 and CO2 contained in the shield gas 8. If the balance of the forming speed at the cathode point of the filler metal 3 and the supply state of the oxide film 32 is maintained, the cathode point always stagnate near the molten drop 31 of the filler metal 3 and generates the arc 2, thereby enabling the formation of beads 6.