A control device is configured to, when the direct current output voltage of a resonant DC-DC converter is of a value that exceeds the maximum value that can be output in a fixed frequency control region, there is a switch from fixed frequency control to frequency modulation control. Because of this, conduction loss and turn-off loss caused by backflow current among semiconductor switching elements of the resonant DC-DC converter are reduced, power conversion efficiency is improved, and the range of voltage that can be output by the resonant DC-DC converter is expanded.