PURPOSE: To subject exactly the welding current in a resistance welding machine to constant current control over a wide range, by providing circuits which control respectively the frequency and pulse width of the control signal for applying the output current of a power rectification circuit to a power inversion circuit.

CONSTITUTION: When a command signal 17 for pressurization and a command signal 18 for current conduction are outputted on the operation of a sequence circuit 16 for a welding machine, a control circuit 11 for pulse width and a control circuit 12 for frequency are controlled by the output of a comparator 10 which compares the output current of a current transformer 8 and the set value of a current setter 9. The controlled output signal of a controlled oscillation circuit 13 is applied to an AND gate 14. The gate 14 outputs the control signal corresponding to an input signal, controls the thyristor of an inverter thereby controlling the output current of a power rectification circuit 1 and the output frequency of a power inversion circuit 3. The welding current is set over a wide range by the above-mentioned device and the constant current control is accomplished exactly to the set value thereof.