A switching power supply apparatus is provided which reduces the time of application of an input voltage to a choke to suppress a peak current flowing through switch elements when a primary or secondary voltage is 0 V (or a voltage near 0 V). A DAB converter or switching power supply apparatus performs phase control to change at least either a first conduction width or a second conduction width of pairs of diagonally-arranged switch elements, constituting a primary full bridge circuit. The first conduction width is a time width in which a pulse for driving the first switch element and a pulse for driving the fourth switch element overlap temporally. The second conduction width is a time width in which a pulse for driving the second switch element and a pulse for driving the third switch element overlap temporally.