A battery state estimating unit (110) estimates an internal state of a secondary battery in accordance with a battery model equation in every arithmetic cycle, and estimates a charging rate (SOC) and a battery current based on a result of the estimation. A parameter estimating unit (130) obtains a battery current (Ib) measured by a sensor as well as the charging rate (SOC) and the battery current (I te ) estimated by the battery state estimating unit (110). The parameter estimating unit (130) estimates a capacity deterioration parameter such that a rate of change in difference (estimation error) between a summed value of an actual current and a summed value of an estimated current with respect to the charging rate (SOC) is minimized. A result obtained by estimating the capacity deterioration parameter is reflected in the battery model by the battery state estimating unit (110).
