To provide a resonance switching power supply in which the output voltage can be controlled to sustain a predetermined level stably while keeping the switching frequency of a pulse current being generated depending on the output state of the power supply.

The resonance switching power supply is constituted of a control circuit 11, a switching circuit 13 including transistors Q1 and Q2, a resonance circuit 14 including an inductor L1 and a capacitor C1, a smoothing circuit 15 including an inductor L2 and a capacitor C2, and an output current detecting load 16 formed of a resistor R wherein a step-down DC/DC converter 12 of a DC input voltage VIN2 for driving the switching power supply is provided on the main power supply side. Output detecting means 17 of the control circuit 11 detects the output voltage VOUT and the output current IOUT to a load by the output terminal TOUT and an output current detecting load 16, controls the output resonance frequency to a predetermined level or above so that countermeasure against ripple is not required even under light load, and controls the DC input voltage VIN2 based on the detected output value VOUT/IOUT so that the output voltage VOUT is sustained at a predetermined level. The converter may be an AC/DC converter.