TECHNOLOGY FOCUS - INORGANIC CHEMISTRY - Preferred Agent: The sulfonated agent is chlorosulfonic acid, fuming sulfuric acid, sulfur trioxide and/or concentrated sulfuric acid.    TECHNOLOGY FOCUS - ORGANIC CHEMISTRY - Preferred Condition: The swelling of high polymer film is carried out at 50-200degreesC. Preferred Solvents: The reactive organic solvent and/or organic solvent for swelling are halogen-containing compounds, and contains 3 or more carbon atoms and 1 or more halogen atoms in molecular structure.    TECHNOLOGY FOCUS - ORGANIC CHEMISTRY - Preferred Agent: The sulfonated agent is chlorosulfonic acid, fuming sulfuric acid, sulfur trioxide and/or concentrated sulfuric acid.    TECHNOLOGY FOCUS - POLYMERS - Preferred Film: The high polymer film is a hydrocarbon-type polymer film consisting of polybenzimidazole, polybenzoxazole, polybenzothiazole, polysulfone, polyether ether ether sulfone, polyaryl ether sulfone, polyphenylene sulfone, polyphenylene oxide, polyphenylene sulfoxide, polyphenylene sulfide, polyphenylene-sulfide sulfone, polyparaphenylene, polyether ketone, polyether ether ketone, polyether ketone ketone and/or cyanic acid ester resin, preferably polyphenylene sulfide. EXAMPLE - Polyphenylene sulfide was melt-extruded to produce a film having thickness of 50 microns. The obtained film was maintained at 150degreesC for 30 minutes, and recrystallized. The obtained film was added to mayonnaise bottle filled with 1-chloro butane. The bottle was maintained at 100degreesC for 4 hours, and cooled to room temperature. The film was removed from the bottle. 1-chlorobutane (671 g) and chlorosulfonic acid (11.26 g) were mixed to obtain chlorosulfonic acid solution. The swollen film was immersed into the obtained solution, and maintained at room temperature for 20 hours. The film was recovered, and cleaned using ion exchange water. The film was dried at 25degreesC and relative humidity of 98%, 80%, 60% and 50%. A proton-conductive polymer film having thickness of 105 microns was obtained. The obtained film had ion exchange capacity of 1.6 milliequivalent, proton conductivity of 3.8x10-2 S/cm and methanol permeability coefficient of 500 micronsol/cm.day.