A switching power supply circuit in which, in order to both provide high power conversion efficiency of a complex resonant converter having a synchronous rectifier circuit and reduce a circuit scale and cost by simplifying the circuit, a synchronous rectifier circuit of a winding voltage detection system is provided on a secondary side of the complex resonant converter, a coupling coefficient is decreased to about 0.8 by setting a gap length in an isolated converter transformer to about 1.5 mm, and numbers of turns of a primary winding and secondary windings are set such that a level of a voltage induced per turn (T) of the secondary windings is 2 V/T. Thus, since magnetic flux density at a core of the isolated converter transformer is decreased to a certain value or lower, a secondary side rectified current can be in a continuous mode even under a condition of heavy load.