A power supply device is mountable to a vehicle and includes: a high-voltage battery; a low-voltage battery of which an output voltage is lower than that of the high-voltage battery, and which can be charged by output power from the high-voltage battery; and a control unit configured to perform charging/discharging control for the high-voltage battery and the low-voltage battery. The control unit performs feeding control for causing the high-voltage battery to supply power to a predetermined on-vehicle device while consumed power is higher than a predetermined value in a state in which a main switch of the vehicle is not on.