TECHNOLOGY FOCUS - INORGANIC CHEMISTRY - Preferred Conductor: The inorganic proton conductor is a heteropolyacid chosen from tungstosilicic acid, tungstophosphoric acid, and molybdophosphoric acid. The solid-polymer-electrolyte material contains 50 wt.% or more of inorganic proton conductor with respect to the weight of polymeric polyfunctional alcohol. The molar ratio (M/C) of poly element (M) and carbon (C) is 0.04 or more.    TECHNOLOGY FOCUS - ORGANIC CHEMISTRY - Preferred Conductor: The inorganic proton conductor is a heteropolyacid chosen from tungstosilicic acid, tungstophosphoric acid, and molybdophosphoric acid. The solid-polymer-electrolyte material contains 50 wt.% or more of inorganic proton conductor with respect to the weight of polymeric polyfunctional alcohol. The molar ratio (M/C) of poly element (M) and carbon (C) is 0.04 or more.    TECHNOLOGY FOCUS - ORGANIC CHEMISTRY - Preferred Agents: The polyfunctional cross-linking agent is polyfunctional carboxylic acid, preferably 2-8C bifunctional carboxylic acid and/or its derivative(s) containing fluorine. The polyfunctional cross-linking agent is polyfunctional isocyanate.Preferred Fuel: 1-3C alcohol, dimethyl ether, and/or aqueous solution or water suspension are used for the fuel of the polymer electrolyte fuel cell.    TECHNOLOGY FOCUS - POLYMERS - Preferred Composition: Alternately, the solid-polymer-electrolyte material comprises a cross-linked material consisting of polymeric polyfunctional alcohol, and an inorganic proton conductor.Preferred Components: The polymeric polyfunctional alcohol is polyvinyl-acetate saponified compound, polyvinyl-acetate copolymer saponified compound, hydroxyalkyl cellulose, chitin, and/or chitosan, preferably polyvinyl alcohol. At least one portion of the hydroxyl group of the polymeric polyfunctional alcohol is modified, preferably urethanated. The polymer having an ionic group has sulfonic acid group, sulfonimide group, sulfate group, and/or phosphonic acid group, preferably sulfonic acid group. The density of the sulfonic-acid group is 0.1-5 mmol/g, preferably 1-3.5 mmol/g. The polymer having an ionic group is styrene sulfonic-acid (co)polymer, acrylamide alkane sulfonic-acid (co)polymer, vinyl sulfonic-acid (co)polymer, and/or perfluoro sulfonic-acid group polymer, preferably polystyrene sulfonic acid, polyacrylamide alkane sulfonic acid, polyvinyl sulfonic acid, polyvinyl alcohol sulfonate, and/or perfluoro sulfonic-acid group polymer.Preferred Property: The total light transmittance of the solid-polymer-electrolyte film is 0-40%.Preferred Condition: The solid-polymer-electrolyte film is manufactured by heat-processing at 105degreesC or more. EXAMPLE - Polyvinyl alcohol having a weight average molecular weight of 124000-186000, tetrafluoro succinic acid, and polystyrene sulfonic-acid-sodium-salt having a weight average molecular weight of 1000000 were mixed with water at a ratio of 67.5/7.5/5/25, to obtain 7 wt.% aqueous solution. The aqueous solution was coated on a glass plate, and cast over two days at 25degreesC and 200 hPa. The obtained cast film was heat-processed at 200degreesC for 1 hour, to obtain a polyvinyl alcohol-type cross-linked film. The obtained film was immersed in 1M hydrochloric acid, and washed with water. The film thickness of the obtained solid-polymer-electrolyte film was 70 microns. The sulfonic-acid-group density of the electrolyte film was 1.3 mmol/g, the methanol transmission amount was 0.8 micronsol/cm2/minute, the ionic conductivity was 4.5 S/cm2, the total light transmittance was 28% as measured by ASTM-D1003, and the swelling rate was 116%.