PURPOSE: To enhance the availability of energy by rectifying an induced electromotive force by a rectifier circuit and storing the rectified power in a condenser circuit.

CONSTITUTION: An article to be welded 3 is held between the two welding electrodes 2a, 2b that are freely opened/closed through the secondary conductor of resistance welding equipment 1, and the welding current is supplied for welding. A power source 4 is constituted of a generating coil 5 by which an induced electromotive force is generated from the welding current flowing through the electrodes 2a, 2b, a rectifier circuit 6 for rectifying the induced electromotive force thus generated, and a condenser circuit 7 for storing the electric power thus rectified. The electric power by the power unit 4 is utilized for the device attached to the resistance welding machine such as an instrument for measuring welding current and a monitor device for discriminating and monitoring normal/ defective condition of welding quality. Thus, it can be used as a power source for equipment using a transmitter.