The present invention provides a separator drastically reducing the short-circuit between electrodes and having satisfactory ionic conductivity on the basis of the use of inorganic materials. The separator 23 has a substrate 23a including a layer formed by using an inorganic material so as to have a plurality of openings, and an inorganic fiber layer 23b laminated on one surface or both surfaces of the substrate 23a so as to cover the openings of the substrate 23a without blocking the openings of the substrate 23a. The thickness of the separator 23 is 100 μm or less.