PURPOSE: To make a pilot transformer unnecessary, by using a starting voltage circuit for TIG welding and a pilot arc generating circuit used for generating plasma arc in common as a rectifier circuit using an auxiliary winding.

CONSTITUTION: By turning a torch switch on, power is supplied to one of taps of primary winding 21a of a main transformer 21. Simultaneously with generation of the secondary output voltage, one of switching contacts 25a or 25b is turned on, and the TIG starting voltage or a pilot arc is generated. When operating as a starting voltage circuit for TIG welding, a changeover switch S1 is set to the TIG side, and an arc is started between a TIG torch and material 29 to be worked. When operating as a pilot arc generating circuit used for generating plasma arc, the changeover switch S1 is set to the cutoff side. A contact 25b is turned on, and a pilot arc is generated between a torch electrode 30a and a narrowing nozzle 30b and shifting to a main arc is effected.