A switching power supply unit includes an N-number (N: an integer of 2 or greater) of transformers; an N-number of inverter circuits; a rectifying smoothing circuit including a {2×(N+1)}-number of rectifying devices, a choke coil, and a capacitor; an additional winding disposed to be interlinked with each of magnetic paths formed in the N-number of transformers; and a driver. In the rectifying smoothing circuit, a (N+1)-number of arms each have two of the rectifying devices, and are disposed in parallel to one another between the pair of output terminals, a secondary winding in each of the N-number of transformers is coupled between adjacent ones of the (N+1)-number of arms to individually form an H-bridge coupling, and the additional winding is coupled in series to one or more of the secondary windings in the N-number of transformers.