The reducing treatment of the negative electrode of which the battery capacity decreases slightly and the internal resistance increases slightly, does not affect the battery performance. Accordingly, by merely supplementing the electrolyte, the battery performance can be recovered. On the other hand, when the level of the degradation of the negative electrode is high, the negative electrode is detached from the battery and is subjected to the reducing treatment so that the negative electrode can be reduced sufficiently without reducing the positive electrode. Consequently, the performance of the negative electrode can be recovered, whereby the battery performance is recovered.