PURPOSE: To prevent the breakdown of a self-extinguishing type element by comparing output currents and a current value obtained by dividing output voltage by the resistance of a heating coil and deciding an operation by a power factor 1.

CONSTITUTION: Output currents from a voltage type inverter flowing through a tank circuit are detected by a current transformer 7, and output currents are converted into voltage by a converter 8. A reference value 9 changing, interlocking with output voltage and an output current value are compared by a comparator 10, thus deciding a power factor of 1. Protection is conducted through operation, such as a bringing to zero of a DC power supply 1, and the stoppage of a gate signal for a self-extinguishing type element, by an output signal from the comparator 10.