To suppress overvoltage applied to a switch element while preventing saturation of a transformer by automatically turning on a switch element connected in parallel with a capacitor when a main switch element is turned off in order to stop a circuit.

A series circuit of a DC power supply 5 and a main switch element 3 is connected across the primary coil 12 of a transformer 4. Furthermore, a series circuit of a capacitor 6 and a parallel circuit of a switch element 8 and a diode 7 is connected in parallel with the primary coil to constitute an active clamp circuit, and a switch element 9 connected in parallel with the capacitor 6 forms a reset circuit. Since the switch element 9 in the reset circuit is turned on with a circuit stop signal, energy stored in the capacitor 6 or circulated from the secondary circuit is discharged. Consequently, overvoltage being applied to the switch element can be suppressed while preventing saturation of the transformer.