A switching power supply which detects a power failure within a short period and maintains the DC output voltage (V.sub.O) for a long time. The switching power supply comprises a first voltage control means which controls a switching means (2) by means of a pulse width modulation means (9) which responds to a first voltage comparator means (8) which compares the DC output voltage (V.sub.O) and a reference voltage (V.sub.R); a second voltage control means having a pulse width modulation means working voltage range expansion means (12) which responds to a second voltage comparator means (11) which compares the DC output voltage (V.sub.O) and a second reference voltage (V.sub.RR); wherein further provision is made of a second switching means (16) for connecting a precharged second capacitor (15) in parallel to the first capacitor (1), in response to the second signal successively outputted by the second voltage comparator means (11) in a short period.