PURPOSE: To make sure of transfer to a plasma arc of a pilot arc by connecting a capacitor between a secondary side midpoint of a transformer and the cathode side of a secondary side rectifier and increasing the no-load voltage between an electrode and a work.

CONSTITUTION: The capacitor 10 is connected via an electromagnetic switch between the secondary side midpoint of the transformer 3 and the cathode side of diodes D2 and D3. A commercial AC power source is converted into DC by a primary side rectifier 1, which is converted into high-frequency AC of about 10-20kHz by an inverter 2 for output control and sent to the transformer 3. This is converted into the voltage suitable for welding and cutting by the transformer 3 and DC electric power converted into DC by the secondary side rectifier 5 is supplied to a plasma torch. At this time, the surge voltage generated at the secondary side of the transformer 3 is peak-charged to the capacitor 10. Accordingly, the peak voltage is held as it is for the output voltage of the rectifier 5 and the no-load voltage between the electrode 8 and the work 7 can be held in a high state.