PURPOSE: To perform welding of high quality with smooth migration of droplet and stable arc under a wide range of welding conditions, by supplying electric current at constant current characteristics in a current ragne smaller than a working point and supplying base current and pulse current to a welding load in the higher range than said range.

CONSTITUTION: Pulses are generated in a basic pulse generating circuit according to the output of a setter 14 for welding conditions. A chopper circuit 12 is driven according to the frequency of said pulses to operate a transistor 3. In this stage, the limiter currents of peak current and base current are controlled by the signal from a current setter 6. The output current is smoothed by a current delay element 7 to a prescribed waveform. When the height of a torch fluctuates and the arc length changes, said length is adjusted by changing the peak current and peak current time until the average voltage fed back with a voltage detector 8 attains the preset value. The peak current is changed by changing the pulse width of the chopper in the period of the peak current time. At the same time, the pulse width is changed by a pulse generating circuit 11.