Abstract: Two welding wires whose current values are individually variable are placed along a groove of steel plates, and two operations are repeated, the first operation including: passing substantially the same current through both welding wires; generating a cathode spot in front of a molten pool by one welding wire's arc on a welding-direction forward side; and cleaning the steel plates' surfaces by the arc, and the second operation including: passing a pulse current having a higher value than that of the welding wire through the other welding wire, so that a cathode spot is generated in the molten pool by each welding wire's arc to newly form a molten pool; and advancing both welding wires in the welding direction to move the cathode spot to the newly-formed molten pool, and at the same time performing welding within an area where oxides on the steel plates' surfaces are removed.