PURPOSE: To prevent internal strapping by calculating the protrusion amount of a separator from a plate based on the easiness to make current flow through the separator between positive and negative electrode plates and the easiness to make the current flow applied in an electrolyte through the separator.

CONSTITUTION: A separator S is inserted between positive and negative electrode plates and the current path between the plates consists of a path I composed of P→A→N, path II composed of P→(section protruded outside the plate)B→N, and path III composed of P→C→N. The optimum value of the protrusion amount DB of the separator S from the peripheral section of the plates P and N is calculated by comparing the electric conductivity in the path III and the electric conductivity of the path I applied directly in an electrolyte through the separator S and is set as shown in the path II. As a result, life performance can be improved by establishing the matching of an assembled element based on the protrusion amount DB of the separator required for prevention of internal strapping at the minimum.