A volume Ve of an electrode group thereof is calculated by Ve = (Sp + Sn) x D/2, where Sp represents an electrode plate area of a positive electrode plate, Sn represents an electrode plate area of a negative electrode plate, D represents the internal dimension of a container in the direction in which the electrode plates of the electrode group are laminated. A ratio (Vp + Vn)/Ve is 0.27 to 0.32, where Vp + Vn is the sum volume of the total pore volume Vp of a positive active material and the total pore volume Vn of the negative active material contained in the electrode group, and Ve is the volume of the electrode group. A ratio Vp/Ve is 0.13 to 0.15, where Vp is the total pore volume of the positive active material and Ve is the volume of the electrode group.