To restrain the degradation of electrode performance caused by improper paste viscosity, restrain reduction in battery capacity caused by decomposition of a thickener at overcharge time, and restrain an excessive increase in battery internal pressure at charging.

Theoretical capacity of a negative electrode is formed so as to become equal to the total of prescribed charging reserve and discharging reserve to theoretical capacity of a positive electrode, and the charging reserve is set not less than a discharging reserve increasing quantity caused by decomposition of a thickener in both electrodes by oxygen gas at over charging. Here, the discharging reserve increasing quantity is regulated by (an electron generating quantity Ah x the total quantity mol of the thickener). By being constituted in this way, oxygen generated in a positive electrode at overcharging reacts with the thickener, and decomposes the thickener, and even if the discharging reserve increases, theoretical capacity of the negative electrode does not become less than the total of theoretical capacity of a positive electrode and the discharging reserve, and an increase in battery internal pressure by a too small quantity of the charging reserve can be prevented.