"it for calculating n.sub.d =n.sub.s -n, where n.sub.s is a preset number of short-circuiting times, and a control means for controlling a welding output of the welding machine by decreasing the welding arc voltage in accordance with the magnitude of n.sub.d, when n.sub.d is larger than zero, and by increasing the welding arc voltage in accordance with the magnitude of .vertline.n.sub.d .vertline.," . . . .