To solve the problem in a conventional battery pack, where a plurality of nickel hydrogen batteries or the like are connected in series, such as in the voltage of each battery cell gradually becomes unequal and that the management takes much time.

A voltage Vi across the terminals of each single battery ECi is detected, and when the maximum value of the dispersion gets over a prescribed value, its discharge switch Ti is closed as to a single battery ECi, where the dispersion is on a certain level or higher, thereby forming a closed circuit passing a discharge resistor Ri. As a result, only the battery ECi where the voltage is high is discharged, and the dispersion of voltage across terminals is equalized. Since there is no need to perform charge separately for each single battery ECi, the circuit is simplified, and leveling can be performed easily. As a result, the rise of reliability and the service life prolongation of a battery pack, where single batteries are combined and a vehicle where this battery pack is mounted, can be realized.