A switching power supply that generates a DC output voltage by conversion of an input voltage coming from a pair of input terminals (T1, T2), and outputs the resulting DC output voltage from a pair of output terminals (T3, T4), the switching power supply comprising: a transformer including a primary winding (Lp1, Lp2) disposed on a side of the pair of input terminals (T1, T2), and a secondary winding (Ls1, Ls2) disposed on a side of the pair of output terminals (T3, T4); a switching circuit disposed on the side of the pair of input terminals (T1, T2), and including first and second switching elements (S1, S2), first and second rectifying elements (D1, D2), first and second capacitive elements (C1, C2), and a first inductor; and a rectifying/smoothing circuit disposed on the side of the pair of output terminals (T3, T4), wherein a first bridge circuit is configured, in the switching circuit, by the first and second switching elements (S1, S2) located in a diagonal arrangement, and the first and second capacitive elements (C1, C2) also located in a diagonal arrangement, the first rectifying element is connected in parallel to the first switching element, the second rectifying element is connected in parallel to the second switching element, and one of the first and second rectifying elements (D1, D2) is connected in forward direction and other is connected in inverse direction, the first inductor is disposed on a connection line between the pair of input terminals (T1, T2) and the first bridge circuit, and the primary winding (Lp1, Lp2) is connected to the first bridge circuit to form a H-bridge configuration, and the secondary winding (Ls1, Ls2) is disposed in the rectifying/smoothing circuit, characterised in that wherein the rectifying/smoothing circuit includes a second inductor, the primary winding (Lp1, Lp2) includes first and second primary windings (Lp1, Lp2) connected in series to each other, and the secondary winding (Ls1, Ls2) includes a first secondary winding magnetically coupled to the first primary winding, and a second secondary winding magnetically coupled to the second primary winding, wherein the rectifying/smoothing circuit includes third and fourth rectifying elements (31, 32), and a third capacitive element (Cout), a second bridge circuit is configured, in the rectifying/smoothing circuit, by an arm including the first secondary winding and the third rectifying element, and another arm including the second secondary winding and the fourth rectifying element, the second inductor is connected to the second bridge circuit to form a H-bridge configuration, and the third capacitive element (Cout) is disposed between a pair of connection lines (LO, LG) which connects the second bridge circuit to the pair of output terminals (T3, T4). a transformer including a primary winding (Lp1, Lp2) disposed on a side of the pair of input terminals (T1, T2), and a secondary winding (Ls1, Ls2) disposed on a side of the pair of output terminals (T3, T4); a switching circuit disposed on the side of the pair of input terminals (T1, T2), and including first and second switching elements (S1, S2), first and second rectifying elements (D1, D2), first and second capacitive elements (C1, C2), and a first inductor; and a rectifying/smoothing circuit disposed on the side of the pair of output terminals (T3, T4), wherein a first bridge circuit is configured, in the switching circuit, by the first and second switching elements (S1, S2) located in a diagonal arrangement, and the first and second capacitive elements (C1, C2) also located in a diagonal arrangement, the first rectifying element is connected in parallel to the first switching element, the second rectifying element is connected in parallel to the second switching element, and one of the first and second rectifying elements (D1, D2) is connected in forward direction and other is connected in inverse direction, the first inductor is disposed on a connection line between the pair of input terminals (T1, T2) and the first bridge circuit, and the primary winding (Lp1, Lp2) is connected to the first bridge circuit to form a H-bridge configuration, and the secondary winding (Ls1, Ls2) is disposed in the rectifying/smoothing circuit, characterised in that wherein the rectifying/smoothing circuit includes a second inductor, the primary winding (Lp1, Lp2) includes first and second primary windings (Lp1, Lp2) connected in series to each other, and the secondary winding (Ls1, Ls2) includes a first secondary winding magnetically coupled to the first primary winding, and a second secondary winding magnetically coupled to the second primary winding, wherein the rectifying/smoothing circuit includes third and fourth rectifying elements (31, 32), and a third capacitive element (Cout), a second bridge circuit is configured, in the rectifying/smoothing circuit, by an arm including the first secondary winding and the third rectifying element, and another arm including the second secondary winding and the fourth rectifying element, the second inductor is connected to the second bridge circuit to form a H-bridge configuration, and the third capacitive element (Cout) is disposed between a pair of connection lines (LO, LG) which connects the second bridge circuit to the pair of output terminals (T3, T4).