To weld a whole section at a time even though a planned weld line is not a closed route by conducting arc retry performance to destroy an oxide film of a work surface with a welding wire when an arc is not generated at a planned welding start point, while maintaining the arc generated, and advanc ing a welding torch from a welding start planned point along a planned weld line.

When an arc 2500 is generated by arc retry performance, its position turns out as an arc start point AS. Next, a welding torch 2000 is made to return from the arc start point AS to a planned welding start point S. During this process, the arc is maintained by the work welding. Succeedingly, a welding treatment is conducted anew from a planned welding start point S along a planned weld line. Thus, the section from the planned welding start point S to the arc start point AS is doubly welded two times of preceding/succeeding.