A DC voltage source, a switch, and an inductor are connected together in series. The inductor is also connected together with a switch, a piezoelectric element in series. Moreover, the switches are connected in parallel with capacitors. By the control circuit, a state in which either of the switches is turned on is repeated through a state in which both of the switches are turned off. Furthermore, in a state in which both switches are turned off, after turning the switch off to enter the state concerned, when the voltage across both terminals of the switch that will be turned on is reduced by the resonance of a resonance circuit composed of the capacitors and the inductor, the switch concerned is turned on.