A control circuit converts power by controlling a phase difference between a switching phase of a plurality of switching elements of a first bridge circuit and a switching phase of a plurality of switching elements of a second bridge circuit such that the control circuit controls the phase difference to be smaller to reduce the output power. When the phase difference reaches a predefined lower limit value in a step-down mode of stepping down the input power, the control circuit controls an on-time of the plurality of switching elements of the second bridge circuit to be shorter while the phase difference is fixed at the lower limit value.