The present invention provides a switching power supply unit and a voltage converting method capable of suppressing a surge voltage applied to an output rectifier element more effectively. Energy in the direction of suppressing reverse voltages (surge voltages) applied to a plurality of rectifier diodes is injected into a rectifier circuit. Reverse voltages applied to the plurality of rectifier diodes are maintained to be lower than a voltage to be inherently applied for a predetermined period. Therefore, rise in the surge voltage is suppressed and rectifier elements (rectifier diodes) having low withstand voltage can be used.