NOVELTY - A solid electrolyte (A1) comprises an aromatic ring in a main chain and a sulfonic acid group bonds to the aromatic ring via a linking group containing a sulfur atom.    USE - As film for fuel cell, membrane and electrode assembly; in fuel cells (claimed) for automobile use, household use, portable device use, as power source of a robot for industrial use or household use, or other toys and games.    ADVANTAGE - The solid electrolyte exhibits excellent ion conductivity, and use of solid electrolyte film as a binder of catalyst layer results in higher cell voltage and gas diffusivity. The use of solid electrolyte as a binder of catalyst layer can advance the battery performance because it enhances electrochemical adhesion between the solid electrolyte and the catalyst layer. By bonding a sulfonic acid group via an alkyl chain containing a sulfur atom, the solid electrolyte does not become water-soluble when the amount of the sulfonic acid group is increased more than ever in order to raise the proton conductivity, to enhance the battery performance. By bonding a sulfonic acid group to an aromatic ring in the polymer main chain via an alkyl chain containing a sulfur atom, the distance between the polymer main chains is widened with the alkyl chain to raise the diffusion performance for oxygen and hydrogen as fuels in a catalyst layer. The solid electrolyte shows a proton conductivity of 0.1 (preferably 0.2) S/cm, e.g. in water at 80degreesC, storage elastic modulus of greater than or equal to0.5 (preferably greater than or equal to1.5) Gpa. As to durability, it shows rate of change of weight and ion-exchange capacity before and after treatment in 30% hydrogen peroxide at a constant temperature of preferably less than or equal to20% (preferably less than or equal to10%); a volume swelling ratio in an ion-exchanged water at a constant temperature of less than or equal to10% (preferably less than or equal to5%); and Tg (glass transition temperature) of greater than or equal to120degreesC (preferably greater than or equal to170degreesC). The solid electrolyte has a stable water absorption coefficient and moisture content. It has a substantially negligible solubility to alcohols, water and mixed solvents. It has a substantially negligible weight loss and figure change when dipped in the solvent.    DETAILED DESCRIPTION - INDEPENDENT CLAIMS are included for the following:    (1) a film (F1), or membrane and electrode assembly comprising the solid electrolyte (A1) (11); and a gas diffusion electrode consisting of a cathode electrode (13) and an anode electrode (12) sandwiching the solid electrolyte;    (2) a fuel cell comprising film (F1); and    (3) producing the solid electrolyte involving either reacting a monomer containing at least one sulfonate group and at least two releasable groups with a comonomer containing at least two releasable groups to form a sulfonated polymer, and removing the releasable group by condensation of the monomer and the comonomer through the reaction; or introducing a halogenomethyl group into an aromatic ring of an aromatic ring-containing polymer, and reacting the aromatic ring-containing polymer with a compound having a group containing a sulfonate group and a sulfur atom.    DESCRIPTION OF DRAWING(S) - The figure shows schematic cross-sectional view of the constitution of a catalyst electrode-bonding film employing the solid electrolyte.    solid electrolyte (11)    anode electrode (12)    cathode electrode (13)    carbon material (12a, 13a)    catalyst metal. (12b, 13b)