A superimposed voltage waveform with a predetermined cycle and amplitude is superimposed on a commanded voltage waveform for a second predetermined time after a first predetermined time elapses within an arcing period in which an arc is produced. The predetermined cycle of the superimposed voltage waveform is shorter than a short-circuit cycle and longer than a control cycle of a welding power supply unit. This suppresses a growth of a weld droplet by being pushed up toward a consumable electrode and suppresses an excessive growth of the droplet over an entire range from a low current range, characterized by frequent short-circuits, to a medium to high current range where short-circuits are less frequent. The droplet is also smoothly transferred to the weld pool in addition to preventing excessive droplet growth. This reduces the generation of spatter, giving a flatter bead appearance.