TECHNOLOGY FOCUS - POLYMERS - Preferred Polymer: The polyimide fiber which comprises polyimide non-woven fabric is crystalline polyimide fiber which consists of thermoplastic polyimide. The polyimide fiber has fibril structure and is obtained by beating short fiber having single yarn size of 0.9-1.7 dtex. The thermoplastic polyimide consists of repeating unit of formula (1).R = tetravalent aromatic residue whose aromatic ring is chosen from monocyclic aromatic, fused polycyclic aromatic, and non-fused polycyclic aromatic bonded directly or by a cross-linking substance;X = bivalent residue chosen from direct bond, hydrocarbon group, carbonyl group, ether group, thio group or sulfonyl group; andY1-Y4 = univalent residue chosen from hydrogen, alkyl, alkoxyl or halogen.Preferred Electrolyte Film: The electrolyte film has mean coefficient of linear expansion of -20 to 20 mum/m.degreesC at 200degreesC or less. EXAMPLE - Polyimide fiber of single yarn size 1.4 dtex was obtained using Aurum PL450 having melting point at 387degreesC and glass transition point at 250degreesC. The polyimide fiber was cut to length of 5 mm and polyimide short fiber was obtained. The polyimide short fiber (in g) (600) was added into polyester group polymer emulsion (6) in 60 l of water and stirred for 1 minute, and slurry for refiner processing was obtained. The slurry was supplied into single disc type refiner and beating processing was carried out for 20 minutes to obtain polyimide short fiber having fibril structure. A polyimide non-woven fabric was produced using the fibrillated polyimide short fiber. The non-woven fabric had fabric weight of 54 g/m2, thickness of 60 mum and porosity of 64%. The fabric was immersed in Nafion perfluoro carbon group sulfonic acid resin solution and dried to obtain a polyimide non-woven fabric whose pores were filled with perfluoro carbon group sulfonic acid-type resin. The polyimide film so obtained was used as electrolyte film. A fuel cell was manufactured using the electrolyte film and electric power generation property under heat retention of 80degreesC was evaluated. Short circuit or leak was not produced during the measurement and the cell had output capability of 0.86 W/cm2.