an inverter circuit comprises a reactor and a capacitor connected in series to a load to supply the load with a series resonance high - frequency power , a secondary winding provided at the reactor , and means for feeding a voltage generated in the secondary winding back to a power supply through switching means of a thyristor . the switching means is made conductive to feed the voltage generated in the secondary winding back to the power supply when supply voltage detecting means detects that a supply voltage exceeds a predetermined voltage .