To provide a resonance switching power supply which works at an operating frequency higher than a resonance one and wherein the lower limit value of the operating frequency needs not be set in advance.

A resonance capacitor 21, a resonance inductance 22, and a primary winding 31 of a transformer 3 are connected in series and this series circuit is connected between a connection between switching devices 11 and 12 and one terminal of a power supply 7. A resonance current detecting circuit 5 detects resonance current Ir in a resonance circuit 2. A control section 6 allows the switching devices 11, 12 to operate at a higher frequency region than the resonance frequency fo of the resonance circuit 2 and turns them off based on a detection signal S3 supplied from the resonance current detecting circuit 5 before the resonance current Ir becomes zero.