PURPOSE: To reduce wear of a welding electrode so as to improve working rate and welding quality and to improve welding efficiency due to no power loss in a diode by applying AC output of an inverter to primary winding wire of a transformer and supplying AC welding current from secondary winding wire.

CONSTITUTION: When an energizing command RUN is activated through a start up circuit 12 and timer 13, a counter 23 starts counting of fixed cycle clock pulse, when the counted value reaches the prescribed number, the counter is overflowed to be zero-cleared, simultaneously a pulse signal A is outputted, the counter again repeats the operation to count from zero. Based on an intensity of the current Im set by a current setter 7, a rectangular wave current reference I* which is changed to positive/negative at each pulse signal, is inputted in a current control part 9, an inverter 16 is controlled through PWM controller 20 and driving part 11 so that the current reference I* and inverter output current I1 coincide. By this method, the AC welding current I2 corresponding to the set current Im from the secondary winding wire of transformer 3 is supplied.