TECHNOLOGY FOCUS - POLYMERS - Preferred Process: The electrolyte film is formed by filling the electrolyte polymer in the pores of the porous base material. Preferred Compound: The polymer having proton-conductive group is a polymer of vinyl sulfonic acid. Preferred Material: The porous base material contains a cross-linked resin composition, which contains polymer (A) including polyethylene with weight average molecular weight of 500000 or more, and polymer (B) having reactive functional group. EXAMPLE - Polymer composition (16 wt.pts) containing powder of Norsorex NB (RTM: ring-opened polymer of norbornene) (in wt.%) (3), TPE 821 (RTM: thermoplastic elastomer) (16), ultra-high-molecular-weight polyethylene (81) with weight average molecular weight of 1500000 and liquid paraffin (84 wt.pts) were melt-kneaded at 160degreesC for 60 minutes, to obtain kneaded material. The kneaded material was inserted between the roll cooled to 0degreesC, and rapidly quenched in the sheet form. The obtained sheet was heat pressed at 115degreesC, to obtain sheet of thickness 0.5 mm. The sheet was then biaxially stretched and subjected to desolvent processing using heptane, to obtain microporous film. The microporous film was heat-processed at 85degreesC for 6 hours and subsequently at 116degreesC for 2 hours to obtain porous base material of thickness 17 mm and porosity 39%. Vinyl sulfonic acid (72), m-sodium divinyl benzene sulfonate (16) and V-50 (RTM: water-soluble azo initiator) (1 mol%) were added to sodium dodecyl benzene sulfonate aqueous solution (0.5), to obtain electrolyte monomer solution for filling. The porous base material was immersed into the obtained electrolyte monomer solution, deaerated, irradiated with visible light for 7 minutes, heated at 80degreesC for 1 hour and polymerized, to obtain electrolyte film. The obtained electrolyte film was immersed in 1 N hydrochloric acid, and ion-exchange was carried out for 2 minutes. Then, excess hydrochloric acid was removed, and vacuum dried at 50degreesC for 5 hours, to obtain proton exchange film. The obtained proton exchange film had thickness of 25 microns, proton conductivity of 0.16 S/cm, swelling rate of 5% and methanol permeability coefficient of 1.2 mmol/cm2/hour.