PURPOSE: To optimize the operating times after the voltage detection of row batteries respectively by identifying whether a service battery is a primary battery or a secondary battery by detecting its internal resistance when a switching means is closed to connect a dummy resistance to a load, and setting a detection voltage for a corresponding row battery.

CONSTITUTION: A switching means 3 is provided for connecting/cutting off a dummy resistor (load) 2 to or form a load for a service battery 1. And the internal resistance of the service battery 1 is detected to identify whether it is a primary battery (alkaline dry cell, for example) or a secondary battery (NiCd battery, for example) is made from the difference of the output voltage drop of the battery 1, when the switching means 3 is closed and the dummy resister 2 is connected to the load of the service battery 1, and a detection voltage for a corresponding row battery is set. Consequently, it becomes possible to optimize the operating times after the voltage detection of row batteries respectively.