A switching power supply unit comprising: a full-bridge-type bridge circuit including four switching elements (S1 S2, S3, S4) and generating an input AC voltage on the basis of a DC input voltage; a transformer having a primary winding and a secondary winding, and transforming the input AC voltage into an output AC voltage; a rectifier circuit provided on the secondary side of the transformer, including a plurality of first rectifier elements, and rectifying the output AC voltage by the plurality of first rectifier elements (4A, 4B), thereby generating a DC output voltage; a surge voltage suppressing circuit connected in parallel with the bridge circuit, and including two device sets each configured of a second rectifier element (D5, D6) connected in a manner of reversed polarity and a first capacitative element, the second rectifier element (D5, D6) and the first capacitive element (C5, C6) being connected in parallel with each other; a resonance inductor provided on the primary side or the secondary side of the transformer, and configuring a first resonance circuit in cooperation with the first capacitative elements; and a driving circuit for driving the bridge circuit, wherein resonance time of the first resonance circuit and recovery time of the first rectifier elements (4A, 4B) are set so as to satisfy the following conditional expression 1 / 4 × [ 2 π × L × C 1 / 2} > Trr 1 where {2πx(LxC) 1/2} is resonance time of one cycle in the first resonance circuit; L is inductance of the resonance inductor; C is capacitance value of the first capacitative element; and Trr1 is recovery time of the first rectifier elements (4A, 4B). a full-bridge-type bridge circuit including four switching elements (S1 S2, S3, S4) and generating an input AC voltage on the basis of a DC input voltage; a transformer having a primary winding and a secondary winding, and transforming the input AC voltage into an output AC voltage; a rectifier circuit provided on the secondary side of the transformer, including a plurality of first rectifier elements, and rectifying the output AC voltage by the plurality of first rectifier elements (4A, 4B), thereby generating a DC output voltage; a surge voltage suppressing circuit connected in parallel with the bridge circuit, and including two device sets each configured of a second rectifier element (D5, D6) connected in a manner of reversed polarity and a first capacitative element, the second rectifier element (D5, D6) and the first capacitive element (C5, C6) being connected in parallel with each other; a resonance inductor provided on the primary side or the secondary side of the transformer, and configuring a first resonance circuit in cooperation with the first capacitative elements; and a driving circuit for driving the bridge circuit, wherein resonance time of the first resonance circuit and recovery time of the first rectifier elements (4A, 4B) are set so as to satisfy the following conditional expression 1 / 4 × [ 2 π × L × C 1 / 2} > Trr 1 where {2πx(LxC) 1/2} is resonance time of one cycle in the first resonance circuit; L is inductance of the resonance inductor; C is capacitance value of the first capacitative element; and Trr1 is recovery time of the first rectifier elements (4A, 4B). where {2πx(LxC) 1/2} is resonance time of one cycle in the first resonance circuit; L is inductance of the resonance inductor; C is capacitance value of the first capacitative element; and Trr1 is recovery time of the first rectifier elements (4A, 4B).