A two-dimensional map for calculating an SOC of a secondary battery is corrected. A battery ECU obtains a voltage index of a secondary battery. The voltage index is, for example, a no-load voltage of the secondary battery. When an amount of change of the calculated no-load voltage from an initial state falls outside a predefined range, the battery ECU corrects the two-dimensional map of initial state which is stored in advance in a storage unit using data obtained by statistically processing a plurality of two-dimensional maps obtained from a plurality of vehicles.