A switching power supply unit comprising: a first bridge circuit of a full bridge type including four first switching elements (S1 to S4) and generating an input AC voltage to a first transformer on the basis of a DC input voltage (Vin); first capacitative elements (C1 to C4) each of which is connected in parallel with a respective one of the first switching elements (S1 to S4); a first inductor connected to the first bridge circuit so as to form an H bridge and configuring a first resonance circuit in cooperation with the first capacitative elements (C1 to C4), said first transformer transforming the input AC voltage to generate an output AC voltage from the first transformer; a rectifier circuit (41, 42) provided on a secondary side of the first transformer, including a plurality of first rectifier elements (41A, 41B), and rectifying the output AC voltage by the plurality of first rectifier elements (41A, 41B), thereby generating a DC output voltage; a pair of second capacitative elements (C5, C6) connected in series, the pair of second capacitative elements (C5, C6) connected in parallel with the first bridge circuit and configuring a second bridge circuit in cooperation with two of said first switching elements (S3, S4), said two first switching elements (S3, S4) being connected in series; a second inductor connected to the second bridge circuit so as to form an H bridge and configuring a second resonance circuit in cooperation with the second capacitative elements (C5, C6); a second transformer for transforming an input voltage from the second bridge circuit to generate an output voltage and supplying the output voltage to the rectifier circuit (41, 42); and a driving circuit for driving the first and second bridge circuits and configured to drive the four first switching elements (S1 to S4) in such a manner that a current which is accumulated by a second resonance operation in the second resonance circuit and flows in a direction of suppressing a surge voltage supplied to the first rectifier element has a phase opposite to that of a current which is accumulated by a first resonance operation in the first resonance circuit and flows in a direction of amplifying the surge voltage. a first bridge circuit of a full bridge type including four first switching elements (S1 to S4) and generating an input AC voltage to a first transformer on the basis of a DC input voltage (Vin); first capacitative elements (C1 to C4) each of which is connected in parallel with a respective one of the first switching elements (S1 to S4); a first inductor connected to the first bridge circuit so as to form an H bridge and configuring a first resonance circuit in cooperation with the first capacitative elements (C1 to C4), said first transformer transforming the input AC voltage to generate an output AC voltage from the first transformer; a rectifier circuit (41, 42) provided on a secondary side of the first transformer, including a plurality of first rectifier elements (41A, 41B), and rectifying the output AC voltage by the plurality of first rectifier elements (41A, 41B), thereby generating a DC output voltage; a pair of second capacitative elements (C5, C6) connected in series, the pair of second capacitative elements (C5, C6) connected in parallel with the first bridge circuit and configuring a second bridge circuit in cooperation with two of said first switching elements (S3, S4), said two first switching elements (S3, S4) being connected in series; a second inductor connected to the second bridge circuit so as to form an H bridge and configuring a second resonance circuit in cooperation with the second capacitative elements (C5, C6); a second transformer for transforming an input voltage from the second bridge circuit to generate an output voltage and supplying the output voltage to the rectifier circuit (41, 42); and a driving circuit for driving the first and second bridge circuits and configured to drive the four first switching elements (S1 to S4) in such a manner that a current which is accumulated by a second resonance operation in the second resonance circuit and flows in a direction of suppressing a surge voltage supplied to the first rectifier element has a phase opposite to that of a current which is accumulated by a first resonance operation in the first resonance circuit and flows in a direction of amplifying the surge voltage.