PURPOSE: To surely perform battery backup by a battery backup circuit such as electronic equipment, etc.

CONSTITUTION: A battery holder 7 is provided with a battery installation detecting terminal c, and whether a battery is installed or not is surely detected making use of the matter that both terminals short-circuits with each other whether the direction is correct or wrong if only a battery 8 is installed between it and a power supply terminal b. Next, the battery holder 7 is provided with a voltage detecting terminal d, and by a comparator 5 the power supply capacity of the battery 8 is judged, and also the judgment of whether the direction of the installation of the battery 8 is correct or wrong is possible, together with the judgment results of the comparator 6 detecting the voltage of the power supply terminal of the battery holder 7.