A battery controller for charge and discharge control of an assembly battery so that a state of charge of each battery cell forming a battery does not deviate from a predetermined usage range. The battery controller controls an assembly battery comprising a plurality of battery cells. The battery controller calculates an actual average charge state CAVE, an average charge state upper limit value CAVEH, an average charge state lower limit value CAVEL, an average charge state ratio S indicating a position of the actual average charge state CAVE between the average charge state upper limit value CAVEH and the average charge state lower limit value CAVEL, and an assembly battery charge state CPACK based on the actual average charge state CAVE, the average charge state upper limit value CAVEH, the average charge state lower limit value CAVEL, and the average charge state ratio S.