A measurement unit (200) measures voltages and currents of battery cells (100). A battery control unit (400) calculates a present estimation value of a residual capacity of the battery cells (100) by integrating the currents. The voltages of the battery cells (100) are set to a reference voltage value V1 serving as a trigger of a process of correcting the estimation value of the residual capacity and an alarm voltage value Va which is a voltage higher than the reference voltage value. In addition, the battery control unit (400) continues discharge of all the battery cells (100), as it is, when an alarm condition in which a voltage of a minimum capacity cell is equal to or less than the alarm voltage value is not satisfied, and outputs a first signal when the voltage of the minimum capacity cell satisfies the alarm condition.