A backup power supply device to be connected to a conduction path from an external power supply to an external load includes a plurality of secondary batteries connected in parallel, a plurality of charging switches that can individually pass and cut off charging power from the conduction path to the plurality of secondary batteries, and a control device for performing discharging control on the plurality of secondary batteries when the power supply voltage of the conduction path drops below a predetermined power failure detection threshold value. When battery voltages of the plurality of secondary batteries drop below a predetermined charging start threshold value, the control device performs charging control with a time difference set between charging start timings of the secondary batteries.