PURPOSE: To obtain welding having high reliability by converting the current measured by a secondary current sensor to equivalent effective welding primary current to compare with optimum welding primary current and controlling a secondary current controller by the differential signal therebetween.

CONSTITUTION: A signal indicating the welding secondary effective current is outputted by a magnetic reluctance sensor 9 and a converter 11. A divider circuit 12 outputs the instantaneous value of the conversion ratio of a transformer 1. A multiplier circuit 13 calculates the welding effective current value from the output signals of the circuits 11 and 12. The primary current values is thus determined. A calculating device 15 calculates the optimum welding primary current from the data given by a keyboard 17 and the measured value of the primary current from a current sensor 16. The output signals of the multiplier circuit 13 and the calculating device 15 are compared by an adjusting device 14 and when the differential signal exceeds the prescribed threshold, the primary current is adjusted so as to restore the value below the threshold. As a result, the value corresponding to the optimum value of the secondary welding current may be obtained at all times at the level of the effective welding secondary current.