PURPOSE: To obtain good weld beads of high quality by controlling the working gas pressure of an welded object to a molten pool to be constant by output signals of a pilot arc voltage comparator.

CONSTITUTION: The pilot are is generated at all times continuously while main arc is being generated. When a nonconsumable electrode 2 is consumed by welding, the distance Lp between nonconsumable electrode 2 and a nozzle electrode 3 becomes longer, and pilot arc voltage Vp becomes higher. Then, the voltage is compared with the pilot arc reference voltage set by a pilot arc reference voltage setting device 11 to detect fluctuation of pilot arc voltage, and the result is made to an output signal and sent to a corrector 13. The corrector 13 makes specified operation to make the distance between the nozzle electrode 3 and the object of welding 5 constant for the reference voltage set by a main are reference voltage setting device 9, and corrects the reference voltage set by the setting device 9 to a rather high value.