TECHNOLOGY FOCUS - POLYMERS - Preferred Properties: The proton conductive polymer has density (D) preferably of 1.55-6 mmol/cm3, more preferably 1.65-6 mmol/cm3, moisture content (MC) of 2-35 wt.%, preferably 5-25 wt.%, more preferably 7-20 wt.%, and 30 wt.% methanol aqueous solution swelling rate of 1.15 or less. The moisture content (MC) is calculated from a formula (5): MC=(water weight in film)/(water absorption film weight)x100.Preferred Process: The polymeric solution is dried and solvent is removed. The proton of sulfonic acid group is substituted by metal, and it is set as -SO3M type (where M is metal having valence of more than 2, preferably 5 or more group metal of periodic table, more preferably barium). The solvent-free polymeric solution is heat-processed at high temperature, and proton substitution is again carried out, to obtain a polymeric electrolyte film.Preferred Polymer: The proton conductive polymer is hydrocarbon-type polymer. EXAMPLE - YPX-100 L (RTM: polyphenylene oxide) (in g) (100) was dissolved in chloroform (1000) at room temperature under nitrogen atmosphere. Chlorosulfonic acid (34 ml) was dropped into the resulting solution, and stirred at room temperature for 30 minutes. The precipitated polymer was separated by filtration, ground, washed with water and vacuum-dried, to obtain sulfonated polyphenylene oxide having bone dry film sulfonic acid group volume density (D1) of 2.98 mmol/g. 1 M hydrochloric acid (0.68) was added to 1,8-bis(trimethoxysilyl) octane (1.35) at 0degreesC, and stirred at room temperature for 30 minutes, to obtain colorless and transparent hydrolyzed substance (S1). 1 M hydrochloric acid (0.37) was added to 5-trimethoxy silyl pentanoic acid trimethyl silyl (1) at 0degreesC, and stirred at room temperature for 30 minutes, to obtain hydrolyzed substance (S2). The hydrolyzed substances (S1 and S2), and sulfonated polyphenylene oxide dimethyl acetamide solution (19.5), were mixed. The resulting liquid was cast on glass plate, and heated at 100degreesC for 3 hours. The plate coated with liquid was immersed in saturated barium chloride aqueous solution at 200-300degreesC under nitrogen gas atmosphere for 1 hour, and barium substitution was carried out. The resulting product was heated at 300degreesC for 10 minutes and cooled. The cooled product was immersed in 1 N hydrochloric acid for more than 3 days, and proton substitution was carried out. The resulting product was washed with water, to obtain a polymeric electrolyte film having thickness of 232 microns. The obtained polymeric electrolyte film had water absorption film sulfonic acid group density (D) of 1.52 mmol/cm3, proton conductivity of 4.9 S/cm2, 30 wt.% methanol aqueous solution swelling rate of 1.3 and moisture content of 50 wt.%, and excellent methanol-proof property.