A secondary side series resonant circuit formed by at least one secondary winding and a secondary side series resonant capacitor is provided together with a secondary side series resonant circuit forming a current resonant converter, whereby a coupling type resonant circuit is formed by magnetic coupling of an isolated converter transformer. For this coupling type resonant circuit to obtain a unimodal characteristic, a gap G formed in a core of the isolated converter transformer PIT is set to about 2.4 mm, and a coupling coefficient k is set to k=0.65 or lower. Thereby, the variable control range of switching frequency can be reduced, and a wide range-ready configuration can be realized. Also, since the variable control range of the switching frequency can be thus reduced, cross regulation of secondary side direct-current output voltage -Eo can be improved.