A switching power supply circuit which can achieve enhancement of the power conversion efficiency is disclosed. In a composite resonance converter wherein a current resonance type converter constructed on the primary side and a primary side partial voltage resonance circuit are combined, a primary side DC input rectification circuit is formed from a full-wave rectification circuit. Further, a gap G of the core of an insulating converter transformer (PIT) is set to 2.0 mm to set the coupling coefficient of the primary winding (N1) and the secondary winding (N2) to approximately 0.81 of a loose coupling state. Further, the numbers of turns of the primary winding (N1) and the secondary winding (N2) are selected so that the induced voltage per one turn of the secondary winding (N2) is 2 V or less. With the configuration, if the leakage inductance (L1l) of the primary winding (N1) and the leakage inductance (L2l) of the secondary winding (N2) are increased, then reduction of the primary side current (I1) in a light load condition can be achieved and the switching output current (IQ1), (IQ2) can be reduced.