To prevent damage to an electrode when an arc is started by actuating a power supply circuit in a manner that the arc is induced by means of a control unit after the predetermined time when an arc starting signal is applied to ionize a protective gas before the output current is validated.

An output controller is turned on by an input device 84, and a protective gas is started to be supplied, whereby a controller 56 actuates a flow rate control meter 90 through a flow rate control circuit 92. At the point when a preliminary flow time is completed, the controller 56 actuates an arc starter circuit 80 so as to generate the arc starting signal. The signal is provided to a power output part 68 during the predetermined time that the arc starting signal ionizes the protective gas. Then, the controller 56 actuates the power circuit 46. When a direct current TIG is chosen, the controller actuates a background circuit 54 and decides whether the arc is induced or not by monitoring a voltage feedback signal via a voltage feedback path 66.