To solve the problem of increasing ripple by including an arrangement for improving power factor in a switching power supply.

In a compound resonance converter combining a partial resonance voltage circuit for a half-bridge coupling system current resonance converter, power factor is improved by a power factor improving circuit 10 of such an arrangement as the primary series resonance current obtained in a primary series resonance circuit is regenerated as power and fed back to a smoothing capacitor Ci (power regeneration system). Under such an arrangement, flux density of a power isolation transformer PIT is set not higher than a predetermined level so that the rectification operation current of a secondary both wave rectification circuit is in a continuous mode regardless of load variation thus suppressing increase in ripple voltage of the secondary DC output voltage caused by power regeneration. Furthermore, the number of components is decreased by sharing a diode performing rectification operation and a diode performing switching operation for a commercial AC power supply. A higher power factor can be attained by an arrangement where a power factor improving capacitor C30 is added to the power factor improving circuit.