After the temperature of a battery rises to a predetermined diagnosis temperature, a charge/discharge control unit controls a charge/discharge circuit to charge or discharge the battery. A current-voltage detection unit detects current and voltage of the battery during a period in which the battery is charged or discharged. A differential voltage calculation unit calculates a differential voltage of the battery on the basis of the detected current and voltage. A deterioration state estimation unit estimates a deterioration state of the battery on the basis of a change in the calculated differential voltage.