To improve the shape of a cross-section in the bead width direction by changing the polarity of a welding power supply so that the polarity becomes a reversed polarity at an oscillating tip part and the polarity becomes a straight polarity in an oscillating center part, and making a wire feed speed increase at the time of the straight polarity while synchronizing with the change of the polarity.

At the time of oscillating welding torches 2 in the groove width direction by using an AC welding power supply whose polarity ratio is changeable, by changing the polarity of the welding power supply so that the polarity becomes a reversed polarity at an oscillating tip part A and so that the polarity becomes a straight polarity in an oscillating center part B, and by making a wire feed speed synchronize with the change of the polarity, the wire feed speed is increased at the time of the straight polarity. Preferably without changing a welding current value, and by changing the time ratio of the polarity of the welding power supply so that the time ratio of the straight polarity becomes 30% or below at the oscillating tip part A and that becomes 50% or over in the oscillating center part B, the wire feed speed is increased when the time ratio of the straight polarity is 50% or over.