To make the performance of unit cells constituting a combined battery equal to each other without overcharging any one of the cells, when the performance of the cells varies.

Variation between SOCs of cells constituting a combined battery is found (S120), and the SOC of the combined battery is controlled so that the SOC may become a target SOC by setting a value SOC1, having high charging/discharging efficiency, as the target SOC when the variation falls within a prescribed range (S160) or a value SOC2 found based on the temperature T of the combined battery at the target SOC, when the variation does not fall within the range (S180 and S190). Since the value SOC2 is set as a chargeable/dischargeable SOC that can reduce the variations between the SOCs of the cells constituting the combined battery more than desired variation, the performance of the cells can be made equal to each other without overcharging any one of the cells.