To provide an active filter type switching power supply, capable of sufficiently exercising the function of increasing power factor by suppressing the occurrence of higher harmonics.

The difference between a shunted voltage Vf of an output voltage Vo and a DC reference voltage Vrd is amplified with a controlling amplifier 22. The product of the control voltage multiplied by an AC reference voltage Vra is obtained as a target value with a multiplier 24. Then, the voltage drop of the resistor Rs of a switching transistor Q is compared with the target value from the multiplier 24 with a comparator 26. When the voltage drop of the resistor Rs reaches the target value, the comparator 26 resets a state register 28, which is set when a choke coil L finishes discharge. This makes the switching transistor Q repeat on and off. The AC reference voltage is a voltage value obtained by converting the input current of a lamp regulator 30 into a voltage with a current transformer CT51 and superposing it on the voltage obtained as the target voltage by dividing with resistors the full-wave rectified current.