To obtain a voltage sufficient to turn on a clamping switch without being affected by characteristics of a control circuit.

A secondary winding 6B that makes it possible to obtain a voltage proportional to a primary winding 6A is wound on a step-up inductor 6. Also, a clamping switch driving circuit 31 is provided that supplies the voltage generated in the secondary winding 6B of the inductor 6 as a turn-on driving signal for the clamping switch 7, when a pair of switching elements 13, 16 are turned on. The supply of the voltage generated in the secondary winding 6B as the driving signal for turning on the clamping switch 7 makes it possible to obtain the turn-on driving signal of a required voltage value.