Provided is a switching power supply unit that includes a pair of input terminals, a pair of output terminals, two transformers, two inverter circuits, a rectifying smoothing circuit, and a driver. The rectifying smoothing circuit includes eight rectifying devices, a first choke coil, a second choke coil, and a capacitance. In the rectifying smoothing circuit, two full-bridge rectifying circuits are provided that each include a first arm and a second arm. The first arm and the second arm each have two of the eight rectifying devices. Secondary windings of the respective two transformers are each coupled to corresponding one of the two full-bridge rectifying circuits to form an H-bridge coupling. Coupling relation of the first arm, the second arm, the first choke coil, the second choke coil, and the capacitance is appropriately defined.