A switching operation of two switching elements is controlled by a controller. Specifically, by adjusting the duty ratio concerning the lower switching element, a capacitor voltage is controlled to a target value. This control is executed not only by simply performing a PI control with respect to the capacitor voltage based on a deviation from the target value, but also by performing feedback control with respect to battery power and output energy. Further, a scheduling factor based on battery voltage, capacitor voltage, and the like is employed to control the gain of the feedback control.