A power supply apparatus comprising: a secondary battery; a timer unit for measuring a time required to charge the secondary battery from a first SOC to a second SOC with constant current, or a time required to discharge the second battery from the second SOC to the first SOC with a certain discharging power; and a determination unit for determining a degraded state of the secondary battery based on results of the measurement by the timer unit, characterized in that the power supply apparatus further comprises: a charging unit for charging the secondary battery; a discharging unit for discharging the secondary battery; and a charging/discharging control unit for controlling charging of the secondary battery by using the charging unit and discharging of the secondary battery by using the discharging unit, wherein the charging/discharging control unit controls the charging unit so that, at a predetermined timing, the secondary battery is charged from the first SOC to the second SOC, or controls the discharging unit so that, at the predetermined timing, the secondary battery is discharged from the second SOC to the first SOC, wherein the secondary battery is charged to a predetermined capacity or more and is then used as a backup power source for supplying power to a load appliance in case of emergency, and wherein the charging/discharging control unit charges the secondary battery to a backup guarantee SOC or more, and varies the backup guarantee SOC so that the capacity of the secondary battery becomes the predetermined capacity or more depending on the degraded state of the secondary battery determined by the determination unit. a secondary battery; a timer unit for measuring a time required to charge the secondary battery from a first SOC to a second SOC with constant current, or a time required to discharge the second battery from the second SOC to the first SOC with a certain discharging power; and a determination unit for determining a degraded state of the secondary battery based on results of the measurement by the timer unit, characterized in that the power supply apparatus further comprises: a charging unit for charging the secondary battery; a discharging unit for discharging the secondary battery; and a charging/discharging control unit for controlling charging of the secondary battery by using the charging unit and discharging of the secondary battery by using the discharging unit, wherein the charging/discharging control unit controls the charging unit so that, at a predetermined timing, the secondary battery is charged from the first SOC to the second SOC, or controls the discharging unit so that, at the predetermined timing, the secondary battery is discharged from the second SOC to the first SOC, wherein the secondary battery is charged to a predetermined capacity or more and is then used as a backup power source for supplying power to a load appliance in case of emergency, and wherein the charging/discharging control unit charges the secondary battery to a backup guarantee SOC or more, and varies the backup guarantee SOC so that the capacity of the secondary battery becomes the predetermined capacity or more depending on the degraded state of the secondary battery determined by the determination unit. a charging unit for charging the secondary battery; a discharging unit for discharging the secondary battery; and a charging/discharging control unit for controlling charging of the secondary battery by using the charging unit and discharging of the secondary battery by using the discharging unit, wherein the charging/discharging control unit controls the charging unit so that, at a predetermined timing, the secondary battery is charged from the first SOC to the second SOC, or controls the discharging unit so that, at the predetermined timing, the secondary battery is discharged from the second SOC to the first SOC, wherein the secondary battery is charged to a predetermined capacity or more and is then used as a backup power source for supplying power to a load appliance in case of emergency, and wherein the charging/discharging control unit charges the secondary battery to a backup guarantee SOC or more, and varies the backup guarantee SOC so that the capacity of the secondary battery becomes the predetermined capacity or more depending on the degraded state of the secondary battery determined by the determination unit.