A porous metal material 10 was filled with an active material slurry, dried, and then rolled to a predetermined thickness. This rolling causes pores 11 formed by a network skeleton to be stretched in the rolling direction (direction represented by the arrow in FIG. 1(a)) to form pores 12 having a shape similar to ellipsoid or deformed ellipsoid having a long axis. Subsequently, the porous metal material was cut in such an arrangement that the longitudinal direction of the pores 12 having a shape similar to ellipsoid or deformed ellipsoid coincides with the crosswise direction of the electrode plate, and then subjected to roller treatment through a series of rollers in the longitudinal direction of the electrode plate. This roller treatment causes numerous cracks 14 to be formed at a very small pitch in the direction parallel to the rolling direction as shown diagrammatically in FIG. 1(c).