The life of a secondary battery is extended and important data is backed up when power trouble, such as an incidental power failure, occurs. When a sudden power failure occurs, a battery supplies backup power to a memory for storing data to be backed up. By using a main switch for turning the main power supply of the apparatus on or off, the supply of the backup power from the battery to the memory is stopped, interlocked with the main switch, when the main switch shuts down the main power supply. This reduces the number charge-discharge cycles of the battery, prevents deterioration of the battery, and prolongs the life of the battery.