To reduce the loss in an output circuit under a light load or no load and prevent generation of a surge voltage and a reverse current when an operation stops.

When a voltage drop across a choke coil 7 decreases and becomes the predetermined value or less, a gate voltage breaking circuit 21 breaks a gate voltage supplied to a commutating MOS FET 6. The loss in the output circuit under the light load or no load is reduced. The commutating MOS FET 6 is turned off and generation of the surge voltage and the reverse current are surely prevented when the operation of a switching element 2 stops.