TECHNOLOGY FOCUS - POLYMERS - Preferred Process: A liquid of electrolyte polymer having proton conductivity is flow-spread on porous membrane such that pores of porous membrane are filled with electrolyte polymer. Alternately, the porous membrane is immersed into liquid of electrolyte polymer, and the pores of porous membrane are filled with electrolyte polymer under reduced pressure. The liquid of electrolyte polymer is melt or solution of electrolyte polymer. Preferred Membrane: The porous membrane has polyimide or polyolefin as main component. Preferred Electrolyte: The electrolyte polymer having proton conductivity, is sulfonated polyether sulfone. Preferred Film: The polymeric electrolyte film has electrolyte layer on its surface. EXAMPLE - 3,3',4,4'-biphenyl tetracarboxylic-acid dianhydride, 3,3',4,4'-biphenyl tetracarboxylic acid and oxydianiline were mixed. The resulting mixture was flow-spread on SUS board. Then, methanol was uniformly applied on the board. The board was immersed in water for 5 minutes. The polyamic-acid film precipitated on the substrate was peeled. The obtained polyamic-acid film was dried and heat-processed to obtain polyimide porous membrane. Sulfonated polyether sulfone polymer was dissolved in N,N-dimethylacetamide, to obtain electrolyte polymer solution. The obtained polyimide porous membrane was immersed in the electrolyte polymer solution, and maintained under reduced pressure atmosphere. The membrane was then arranged at glass plate, and vacuum-dried at 150degreesC for 2 hours. The formed film was then peeled, and heat-processed at 200degreesC for 30 minutes. The film was then subjected to ion exchange treatment, to obtain polymeric electrolyte film of 45 microns thickness. The obtained film had proton conductivity of 8.5x10-2 S/cm and methanol permeability coefficient of 0.16 m2h/kgmicrons at 60degreesC and 95% relative humidity. The cross-section of film was analyzed by scanning electron microscope. The result showed that the sulfonated polyether sulfone polymer layer was uniformly formed on both surfaces of film. A fuel cell excellent in electric power generation ability was formed using the obtained polymeric electrolyte film.