A device which judges a change of a capacity balance between a cathode and an anode of a secondary battery in a non-destructive manner is provided. The secondary battery has one of the capacities of the cathode and the anode greater than the other capacity. A battery ECU calculates an internal resistance (DCIR) of the secondary battery based on a current and a voltage of the secondary battery. When the calculated internal resistance (DCIR) is higher than the initial state in a low SOC region or a high SOC region, the battery ECU judges that the capacity balance between the cathode and the anode of the secondary battery has changed.