A switching power supply unit capable of simplifying its structure by reducing the number of parts for wiring is provided. A rectifier circuit has a diode inserted between a first secondary winding and a second secondary winding to form a series circuit therewith, and a diode inserted between a third secondary winding and a fourth secondary winding to form a series circuit therewith. A smoothing circuit has a first choke coil inserted between an input section connected to a center tap and an output terminal, and a second choke coil inserted between an input part connected to a junction point and an output terminal. A first path and a second path are formed of a wiring member made of a single sheet metal, and a third path and a fourth path are formed of a wiring member made of another sheet metal.