A switching power supply apparatus is configured of, as a primary-side circuit, a synchronous rectification step-up/down chopper circuit including a main switching element (14), a commutation element (16), a choke coil (18), and a buffer capacitor (20). As a secondary-side circuit, to a secondary winding provided to the choke coil, a series circuit including a rectifying element and an output capacitor is connected. An inductance of the choke coil is set to have a predetermined value so that a choke coil current IL flowing through the choke coil flows in both of a positive direction and a negative direction across zero in one cycle of switching when an output current is equal to or smaller than a maximum output current. A switching control circuit performs control of complementarily turning the main switching element and the commutation element ON/OFF in a switching cycle by providing a deadtime, thereby performing soft switching.