A switching power supply unit includes input terminals, output terminals, first to third primary windings and first to third secondary windings, a switching circuit, a rectifying smoothing circuit, and a driver. In the switching circuit, first and second switching devices, third and fourth switching devices, and first and second capacitors, coupled in series to one another, are disposed in parallel between the input terminals. In the rectifying smoothing circuit, first to third arms, each having two of rectifying devices disposed in series, are disposed in parallel between the output terminals, the first secondary winding is coupled between the first and the second arms to form an H-bridge coupling, the second and the third secondary windings are coupled between the second and the third arms to form an H-bridge coupling, and a choke coil is disposed between the first to the third arms and an output capacitor.