To realize power saving and increase power supply time for the external apparatus requiring continuous power supply by a method, wherein when input operation that a load which needs power supply only for a constant term at a specific time is used is not performed, the power supply to the load is turned off.

In the case of service interruption or the like, a changeover means 5 is switched to the inverter 4 side, and an AC power source is supplied from the inverter 4. A load input detecting means 7a detects that a power source is changed from a commercial power source 2 to a secondary battery 3, but an on signal is transmitted to a specified output control means 6 similar to the normal times. The load input detecting means 7a monitors the operating state of input device as a keyboard. When the input operation that a load which needs power supply only at a specific time is used is not performed for a fixed time, the specific output control means 6 is so controlled that the output to the load is turned off.