To adjust reinforcement of weld to have a prescribed height while maintenance of speed in high speed welding and prevention of undercut are contrived by using multiple electrodes.

In arc welding using multiple electrodes in a groove 2 between oppositely facing base materials 1, 1, a wire-like electrode 4 preceding in the welding direction is energized by the first required current necessary for securing a required penetration depth, while a succeeding belt-like electrode 5 is used longitudinally along the welding direction and energized by the second required current necessary for securing a required deposited metal quantity.