It calculates a capacity difference ESOC between a reference remaining capacity SOCV based on an open-circuit voltage and a final remaining capacity SOC in a previous one calculation cycle. It calculates a proportional correction amount and an integral correction amount according to a proportional-integral control by using this capacity difference ESOC, and requires a final remaining capacity SOC by adding a remaining capacity SOCI based on a current integration to these correction amounts. Thus, while making good use of the remaining capacity based on the battery open-circuit voltage, it can avoid the influence of the load fluctuation and require a remaining capacity always stable and accurate.