TECHNOLOGY FOCUS - ELECTRICAL POWER AND ENERGY - Preferred Method: The vicinity of the film(s) is pressure-reduced to 600 hPa or less by the pressure controller. The film is extended in the width direction in the reduced-pressure drying process while conveying and drying the film with both ends held by a holder. While conveying and supporting the extended film with the rollers, the film is dried.    TECHNOLOGY FOCUS - ORGANIC CHEMISTRY - Preferred Solvent: The organic solvent is a mixture of poor solvent (10-100 wt.%) of the solid electrolyte, and a good solvent. The good solvent contains 1-5C alcohol and the poor solvent contains dimethyl sulfoxide.    TECHNOLOGY FOCUS - POLYMERS - Preferred Electrolyte: The solid electrolyte is a hydrocarbon group polymer, preferably an aromatic polymer having sulfonic acid group. The aromatic polymer is a copolymer consisting of structural units of formulae (I,II) and of formula: -(-O-Z-O-)-.X=H;Y'=SO2;Z=group of formula (a or b);andn/(m+n)=0.1-0.5. EXAMPLE - A graft copolymer was obtained by polymerizing (in weight parts) acrylonitrile (21), 4-(4-(4-pentyl cyclohexyl) phenoxy methyl) styrene (62), t-dodecyl thiol (0.5), and cumene hydroperoxide (3). 1% sulfuric acid was added to the copolymer, solidified at 90degreesC, and dried after water-washing. The graft copolymer (100) was dissolved in methylene chloride (1300), and concentrated sulfuric acid (13) was added slowly, maintaining the solution temperature at 0degreesC or less. The mixture was stirred for 6 hours, and the precipitate was dried to obtain sulfo polyacrylonitrile styrene-butadiene rubber. A dope was prepared by mixing the rubber component (100) and N,N-dimethyl formamide (400). The dope was flow-spread and dried at 80-120degreesC, to obtain a spread film having a residual solvent amount of 30 wt.%. The spread film was peeled off, dried to a solvent content of 15 wt.%, immersed into water/methanol mixture, and dried to obtain a solid-electrolyte film. The obtained solid-electrolyte film had a thickness of 51 microns, a residual solvent amount of 0.5 wt.%, and an ionic conductivity of 0.13-0.14 S/cm. The electrolyte film provided fuel cell having a power density of 0.56-0.58 W/cm2.