The internal resistance related value which is related to the internal resistance of a secondary battery is compared with a previously obtained relation between the internal resistance related value and battery condition to judge the battery condition of the secondary battery. Since the internal resistance related value is a value related to the internal resistance which closely depends on the battery condition, the battery condition can be judged in detail based on the above relation. And the internal resistance related value can be obtained more speedily with a predetermined method. On the other hand, when the level of the degradation of a negative electrode is low, an electrolyte is supplemented, and when the level of the degradation of the negative electrode is high, a reducing agent is added to the electrolyte to regenerate the secondary battery. With this regenerating method, the performance of the negative electrode can be recovered without degrading a positive electrode.