PURPOSE: To prevent the fault of a computer system serving as a load by switching a back-up battery to a saving battery for supply of power and also transmitting a saving process start signal to a load device when a power failure of an external power supply is detected together with the voltage drop of the saving battery.

CONSTITUTION: A detection circuit 11 is provided to detect the power failure of an external power supply together with a saving battery 13 having the power capacity enough to withstand the saving processes of several times carried out at the side of a load device, and a detection circuit 15 which detects the voltage drop of a back-up battery 9. Then the battery 9 is switched to the battery 13 for supply of power when the power failure of the external power supply is detected together with the voltage drop of the battery 13. At the same time, a saving process start signal is transmitted to a load device. Thus it is possible to prevent the fault of a computer system, etc., which serve as a load.