An SOC of each battery cell is periodically detected, and an SOCmin and an SOCmax are determined. Battery cells having the SOCs larger than SOCmin+α are selectively discharged. After an elapse of a preset equalization processing time period, discharge of all the battery cells is stopped. The equalization processing time period is set based on a rate of change in the SOC of the battery cell subjected to discharge and a rate of change in the SOC of the battery cell not subjected to discharge such that a magnitude relationship between the SOC of the battery cell having the SOCmin and the SOC of another battery cell is not reversed.