Disclosed is a reinforcing sheet comprising glass fibers and an organic binder attached to the glass fibers and having voids in which a solid electrolyte is to be filled. The organic binder comprises: (i) an organic polymer which does not contain any element other than carbon, hydrogen and fluorine; or (ii) an organic polymer which has the main chain and a side chain, wherein the main chain comprises a perfluoroalkylene and the side chain has a sulfonate group or a carboxylate group attached to the terminal of at least a part thereof. The reinforcing sheet is suitable for the improvement in durability and the maintenance of chemical stability of a solid electrolyte membrane.
