VEHICLE-MOUNTED BACKUP DEVICE

A device capable of switching the supply source to a power storage unit without interrupting power supply to a power supply target even in the case where power supply from a power source unit cuts out is realized with a simple configuration. A backup device includes a discharging circuit that steps up or steps down a voltage applied to a power storage unit-side conduction path and applies the resulting voltage to an output-side conduction path, a control unit () that controls the discharging circuit to apply, to the output-side conduction path, a predetermined target voltage that is lower than a voltage that is applied to a power path in the case where a power source unit is fully charged, and a diode (element unit) that is provided between the power path and the output-side conduction path.