Abstract: In a switching power source, by alternately turning on first and second MOS transistors, a switching current flows to a primary winding of a transformer, and an alternate current is transferred to a secondary side. The alternate voltage caused in a secondary winding is full-wave rectified by first and second diodes. Resonance elements (capacitor and coil) are connected to a tertiary winding coupled to an isolation transfer, thereby enabling the primary side to be set into a resonated state and to be set to a current resonance type switching power source. Since a leakage inductance of the transformer is not set to a resonance element, an interval between the windings of the isolation transformer can be closely coupled. Since a resonance current is not directly turned on/off, a peak value of the primary current is decreased and switching loss and loss due to an on-resistance can be decreased. The efficiency of a current resonance type switching power source is thus improved.