To provide a switching power supply capable of controlling the boosting of a voltage when an AC input voltage is low, and reducing an input electric power.

A switching power supply is designed to apply an AC input voltage from a rectifier diode D1 and a limit resistance R1 to a constant voltage diode D2, and to monitor level of the AC input voltage by the constant voltage diode D2. If an input voltage is higher than the constant voltage diode D2, an NPN transistor Q2 will turn on, and then a third resistor R3 connected in parallel to a voltage dividing resistor R5 will be conducting, thus increasing the set voltage of an active filter. If an input voltage is substantially low, the NPN transistor Q2 will not turn on, and the set voltage of the active filter is decreased down to a voltage set by the voltage dividing resistors R4, R5. As a result, the losses of a switching element Q1, an inductor L1, a diode D3 are reduced.