An uninterruptive power supply supplies electric power to a computer by use of a battery when the supply of electric power by a main power supply is interrupted. The system termination processing section detects the outage of the main power supply when the period of interruption of the supply of electric power from the main power supply to the uninterruptive power supply has exceeded a preset period, monitors the remaining capacity of the battery in the uninterruptive power supply at least when the outage is detected, gradually effects the system terminating process to the shutdown of the power supply of the system according to the degree of importance based on the remaining capacity of the battery, and controls the stage of the system terminating process according to a variation in the remaining capacity of the battery when the remaining capacity of the battery is changed during the system terminating process.