Abstract: Circuit groups for preventing a battery from being deteriorated in efficiency due to a charging and discharging of each of secondary batteries is incorporated in a battery protection circuit for protecting a plurality of batteries connected in series to thereby extend a life of the battery. The battery protection circuit incorporates therein power down means for extending a time in which a voltage of a remaining capacity becomes 0V as much as possible by causing a current to flow only in the minimum circuit when the battery is overdischarged. Also, when a trouble such as a momentary overcurrent or the like is detected, the circuit can be prevented from set in the power down mode.