In a backup device, a determination unit determines a first target voltage value of a second power supply unit when a starter switch for starting a vehicle is in an OFF state, so as to be lower than a second target voltage value of the second power supply unit when the starter switch is in an ON state, based on the second target voltage value, a value indicating the charging capability of the charging circuit, and a predetermined time limit. A control unit causes the charging circuit to perform the charging operation upon the starter switch being switched to an ON state, such that the charged voltage of the second power supply unit reaches the second target voltage value, and causes the discharging circuit to perform the discharging operation upon the starter switch being switched to an OFF state, such that the charged voltage reaches the first target voltage value.