A switching circuit is configured to switch on/off a current that flows through a resonance circuit constituted by a primary winding of a transformer, a capacitance element, and an inductor that are connected in series. Electric power that is induced in a secondary winding of the transformer is rectified by a rectifier circuit. A switch is connected in parallel with the capacitance element. A control unit is configured to control the switching circuit and the switch. The control unit is configured to select a first operation in which an operation frequency of the switching circuit is controlled by switching off the switch, and select a second operation in which a period during which a current flows from the switching circuit to the primary winding of the transformer is controlled by switching on the switch.