A switching power supply device which includes an input filter capacitor and a switching device, includes: a transformer comprising a core of the transformer and a first input winding, the first input winding being wound around the core of the transformer, connected between one terminal of the input filter capacitor and one terminal of the switching device; a first capacitive unit connected between both terminals of the first attenuation winding part of the transformer, and including at least one device including a first capacitor; and a clamp unit configured to limit a peak voltage generated by the first input winding when the switching device is turned off during the switching operation of the switching device, the clamp unit including a rectification diode.