In the switching power supply circuit disclosed in the present invention, the DC voltage is supplied to the primary winding in the converter transformer and the AC voltage is generated. First and second series resonant circuit are formed, and the resonant frequency of the first and second series resonant circuits are set to be substantially equal to each other. A parallel resonant circuit is formed, and the resonant frequency of the parallel resonant circuit is set higher than the resonant frequencies of the first and second series resonant circuits. The secondary winding includes first and second secondary windings so that currents are extracted from the first and second secondary windings, respectively, and a ratio of the number of turns of the first secondary winding to that of the second secondary winding is defined so that magnitudes of the currents are substantially equal to each other.