A switching power supply unit is provided, in which core loss in a transformer or a level of heating due to AC resistance of the transformer can be reduced. A switching power supply unit has a transformer, an inverter circuit and a rectifier circuit. In a secondary side of the transformer, secondary wirings are connected to each other. In a primary side of the transformer, primary wirings are connected in series to each other, and connected to the inverter circuit to allow them to be driven in a time-divisional manner in phases opposite to each other in response to operation of the inverter circuit. The rectifier circuit has diodes connected to the secondary wirings to allow the secondary wirings to be driven in a time-divisional manner in phases opposite to each other in response to operation of the inverter circuit.