A separator for electricity storage devices which comprises: a base comprising a porous film; and a thermoplastic polymer disposed on at least one surface of the base. The thermoplastic polymer has a dispersion (sigma2), defined by the following numerical equation using the areas (Si) of Voronoi polygons obtained by Voronoi tessellation, of 0.01-0.7. (In the equation, Si is the measured area of each Voronoi polygon, m is an average of the measured areas of the Voronoi polygons, and n is the total number of the Voronoi polygons.)