A switching power supply device (1, 1A, or 1B) comprising: a pair of input terminals (T1 and T2) configured to receive an input voltage (Vin); a pair of output terminals (T3 and T4) configured to output an output voltage (Vout); N number of transformers (31 to 3n) including respective primary-side windings (311 to 3n1) and respective secondary-side windings (312 (312a and 312b) to 3N2), the N being an integer of 2 or greater; N number of inverter circuits (2 (21 and 22) or 2B (21 to 2N)) disposed in parallel to each other between the pair of input terminals (T1 and T2) and the primary-side windings (311 to 3N1), the N number of inverter circuits (2 (21 and 22) or 2B (21 to 2N)) each including switching elements (S1 to S4), wherein each inverter circuit is configured to convert the input voltage (Vin) applied between the input terminals (T1 and T2) to a predetermined pulse voltage or an alternating voltage with a predetermined voltage value (Va or Vb) and to provide the predetermined pulse voltage or the alternating voltage at the predetermined voltage value (Va or Vb) to the respective primary-side windings (311 to 3N1) of the N number of transformers depending on the state of its switching elements (S1 to S4); a rectifying and smoothing circuit (4 or 4A) disposed between the pair of output terminals (T3 and T4) and the secondary-side windings (312 (312a and 312b) to 3N2), the rectifying and smoothing circuit (4 or 4A) including a rectifying circuit including a plurality of rectifying elements (41 and 42, or 411, 412, 421, and 422) and a smoothing circuit including a capacitor (Cout) disposed between a choke coil (Lch) and the pair of output terminals (T3 and T4); and a driver configured to perform switching driving that controls operations of the respective switching elements (S1 to S4) in the N number of inverter circuits (2 (21 and 22) or 2B (21 to 2N)), and set a voltage (Vd) to be supplied to the smoothing circuit at successive voltage levels (N + 1) from a 0 level to an N level by controlling operations of the switching driving of the N number of inverter circuits (2 (21 and 22) or 2B (21 to 2N)) characterised in that N ≥ 2 and the plurality of the rectifying elements comprises two or four rectifying elements regardless of a value of the N. a pair of input terminals (T1 and T2) configured to receive an input voltage (Vin); a pair of output terminals (T3 and T4) configured to output an output voltage (Vout); N number of transformers (31 to 3n) including respective primary-side windings (311 to 3n1) and respective secondary-side windings (312 (312a and 312b) to 3N2), the N being an integer of 2 or greater; N number of inverter circuits (2 (21 and 22) or 2B (21 to 2N)) disposed in parallel to each other between the pair of input terminals (T1 and T2) and the primary-side windings (311 to 3N1), the N number of inverter circuits (2 (21 and 22) or 2B (21 to 2N)) each including switching elements (S1 to S4), wherein each inverter circuit is configured to convert the input voltage (Vin) applied between the input terminals (T1 and T2) to a predetermined pulse voltage or an alternating voltage with a predetermined voltage value (Va or Vb) and to provide the predetermined pulse voltage or the alternating voltage at the predetermined voltage value (Va or Vb) to the respective primary-side windings (311 to 3N1) of the N number of transformers depending on the state of its switching elements (S1 to S4); a rectifying and smoothing circuit (4 or 4A) disposed between the pair of output terminals (T3 and T4) and the secondary-side windings (312 (312a and 312b) to 3N2), the rectifying and smoothing circuit (4 or 4A) including a rectifying circuit including a plurality of rectifying elements (41 and 42, or 411, 412, 421, and 422) and a smoothing circuit including a capacitor (Cout) disposed between a choke coil (Lch) and the pair of output terminals (T3 and T4); and a driver configured to perform switching driving that controls operations of the respective switching elements (S1 to S4) in the N number of inverter circuits (2 (21 and 22) or 2B (21 to 2N)), and set a voltage (Vd) to be supplied to the smoothing circuit at successive voltage levels (N + 1) from a 0 level to an N level by controlling operations of the switching driving of the N number of inverter circuits (2 (21 and 22) or 2B (21 to 2N)) characterised in that