To realize much higher power conversion efficiency within a range from the heavy load to the low load as a composite resonant converter provided with a current resonant converter.

The current resonant converter of a full bridge coupling constituted with four switching elements Q1-Q4 on the primary side and a primary side partial voltage resonant circuit are combined in the composite resonant converter. A primary side DC input rectification circuit is constituted with a full wave rectification circuit. Further, the gap G of cores of an insulating converter transformer PIT is made to be 2.0 mm. A coupling coefficient of a primary winding N1 and a secondary winding N2 is set to loose-coupling of about 0.81. Also, the number of windings of the primary winding N1 and the secondary winding N2 is selected so as to make the induced voltage per one turn of the second winding N2 to become 2V or lower. By constituting in such a manner, the leakage inductance L1l of the primary winding N1 and the leakage inductance L2l of the second winding N2 are increased. A primary side current I1 during the low load is reduced. As a result, switching output currents IQ1, IQ2 can be reduced.