In a switching power source, by alternately turning on the first and second MOS transistors, a resonance current flows to a primary winding of a transformer, and an alternate current is transmitted to a secondary side. With respect to an alternate signal generated in a secondary winding, a gate voltage is applied so that the third MOS transistor and the fourth MOS transistor are conductive, respectively, while the polarity of the alternate signal is positive. Rectifying currents are flowed to a capacitor and a full-wave rectification is executed. Since a control signal for synchronous control is supplied from a signal source to control the first and second switching elements via transformers, a switching timing can be accurately coincided with an on/off timing of a synchronous rectifying device by a simple circuit. The efficiency of a current resonance type switching power source is thus improved.