A secondary battery comprises a single-plate lead acid battery (Va), a positive electrode capacitor layer and a negative electrode capacitor layer, which are formed on the respective surfaces of a positive electrode plate, and a negative electrode plate. Each of the positive electrode plate and negative electrode plate contains an active material, to have a configuration in which activated carbon has a concentration gradient. The electronic resistance between a collector terminal and the center of a portion where activated carbon is present at a high concentration is lower than the electronic resistance between the collector terminal and the center of a portion where activated carbon is present at a low concentration. In the single-plate lead acid battery, charges are accumulated in the positive electrode capacitor layer and in the negative electrode capacitor layer.