A switching power supply unit (1, 1A, 1B, 1D), comprising: a pair of input terminals (T1, T2) that receives an input voltage (Vin); a pair of output terminals (T3, T4) that outputs an output voltage (Vout); an N-number of transformers (31, 32) each having a primary winding (311, 321) and a secondary winding (312, 322), where N is an integer of 2 or greater; an N-number of inverter circuits (21, 22) each including a switching device (S1, S2, S3, S4), and disposed in parallel to one another between the respective primary winding (311, 321) and the pair of input terminals (T1, T2); a rectifying smoothing circuit (4, 4C, 4D, 4E) including a {2 × (N + 1)}-number of rectifying devices (411, 412, 421, 422, 431, 432), a choke coil (Lch), and a capacitor (Cout) disposed between the pair of output terminals (T3, T4), the rectifying smoothing circuit (4, 4C, 4D, 4E) being disposed between the secondary winding (312, 322) and the pair of output terminals (T3, T4); an additional winding (332, 342) disposed to be interlinked with each of magnetic paths (B1, B2) formed in the N-number of transformers (31, 32); and a driver that performs a switching drive to control operation of the switching devices (S1, S2, S3, S4) in the N-number of inverter circuits (21, 22), wherein in the rectifying smoothing circuit (4, 4C, 4D, 4E), a (N + 1)-number of arms each have two of the rectifying devices (411, 412, 421, 422, 431, 432) disposed in series to one another in a same direction, and are disposed in parallel to one another between the pair of output terminals (T3, T4), the secondary winding (312, 322) in each of the N-number of transformers (31, 32) is coupled between adjacent ones of the (N + 1)-number of arms to individually form an H-bridge coupling, and the additional winding (332, 342) is coupled in series to one or more of the secondary windings (312, 322) in the N-number of transformers (31, 32), and the choke coil (Lch) is disposed between the (N + 1)-number of arms and the capacitor (Cout). a pair of input terminals (T1, T2) that receives an input voltage (Vin); a pair of output terminals (T3, T4) that outputs an output voltage (Vout); an N-number of transformers (31, 32) each having a primary winding (311, 321) and a secondary winding (312, 322), where N is an integer of 2 or greater; an N-number of inverter circuits (21, 22) each including a switching device (S1, S2, S3, S4), and disposed in parallel to one another between the respective primary winding (311, 321) and the pair of input terminals (T1, T2); a rectifying smoothing circuit (4, 4C, 4D, 4E) including a {2 × (N + 1)}-number of rectifying devices (411, 412, 421, 422, 431, 432), a choke coil (Lch), and a capacitor (Cout) disposed between the pair of output terminals (T3, T4), the rectifying smoothing circuit (4, 4C, 4D, 4E) being disposed between the secondary winding (312, 322) and the pair of output terminals (T3, T4); an additional winding (332, 342) disposed to be interlinked with each of magnetic paths (B1, B2) formed in the N-number of transformers (31, 32); and a driver that performs a switching drive to control operation of the switching devices (S1, S2, S3, S4) in the N-number of inverter circuits (21, 22), wherein in the rectifying smoothing circuit (4, 4C, 4D, 4E), a (N + 1)-number of arms each have two of the rectifying devices (411, 412, 421, 422, 431, 432) disposed in series to one another in a same direction, and are disposed in parallel to one another between the pair of output terminals (T3, T4), the secondary winding (312, 322) in each of the N-number of transformers (31, 32) is coupled between adjacent ones of the (N + 1)-number of arms to individually form an H-bridge coupling, and the additional winding (332, 342) is coupled in series to one or more of the secondary windings (312, 322) in the N-number of transformers (31, 32), and the choke coil (Lch) is disposed between the (N + 1)-number of arms and the capacitor (Cout).