TECHNOLOGY FOCUS - ELECTRICAL POWER AND ENERGY - Preferred Method: The film is softened by a temperature controller which controls the temperature of the flattening process, and/or a moisture controller which controls the humidity at the film vicinity. The temperature controller is a roller which controls the temperature of the film contacting the roller. Water vapor is supplied to the vicinity of the film by the moisture controller, and humidity is set tot 50% or more. The roller comprises a cooling roller arranged downstream from the heating rollers arranged along the conveyance path.Preferred Conditions: The temperature (t1) of the film at the peripheral surface of the heating roller with respect to the glass transition temperature (Tg) of the solid electrolyte, is TgdegreesC to (Tg+50)degreesC and ThdegreesC to (Th+50)degreesC, where Th is the heat-deformation temperature (Th) of the solid electrolyte. The temperature (t2) of the film at the peripheral surface of the cooling roller is TgdegreesC or less or ThdegreesC or less.    TECHNOLOGY FOCUS - ORGANIC CHEMISTRY - Preferred Solvent: The weight of the organic solvent in the film during starting of the flattening process is 5 wt.% or less. The organic solvent is a mixture of poor solvent of the solid electrolyte, and good solvent. The weight proportion of the poor solvent is 10-100%. 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 torn 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 excellent curl resistance 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.