A switching power supply circuit includes a primary side series resonant circuit forming a current resonant converter, and a secondary side series resonant circuit formed by at least a secondary winding and a secondary side series resonant capacitor, whereby a coupling type resonant circuit is formed by magnetic coupling of an isolated converter transformer. In order to obtain a unimodal characteristic for this coupling type resonant circuit, the total coupling coefficient of the isolated converter transformer is set to 0.65 or lower. For power factor improvement, a power factor improving circuit of a power regeneration system or a voltage feedback system is provided.