TECHNOLOGY FOCUS - ORGANIC CHEMISTRY - Preferred Electrolyte: The fluorine-type polymer electrolyte is of formula (1).(CF2CF2)a-(CF2-CF(-O-(CF2-CF(CF3))b-O-(CF2)f-X4))g or (CF2CF2)a-(CF2-CF(-O-(CF2)f-X4))g (1)a=greater than or equal to0 and less than 1;g=greater than 0 and less than or equal to1;a+g=1;b=1-3;f=1-8;andX4=-COOH, -SO3H,-PO3H2 or -PO3H.Preferred Medium: The fluid medium further contains aprotic solvent. The quantities of a protic solvent and aprotic solvent with respect to total mass of both the solvents are 0.5-99.5 mass% and 99.5-0.5 mass%, respectively.Preferred Solvent: The protic solvent is water.    TECHNOLOGY FOCUS - POLYMERS - Preferred Film: The polymeric electrolyte film has sea/island structure having cross-sectional area of 15 micronsx15 microns, when observed by transmission electron microscope along thickness direction. A reinforcement material is maintained at surface or interior of electrolyte film.Preferred Properties: The content of basic polymer is preferably 0.01-10 mass% with respect to sum total mass of components (a,b). The ion exchange capacity of the polymeric electrolyte film is 1-1.5 milliequivalent/g. The thickness of polymeric electrolyte film is 2-150 microns. The haze value of the polymeric electrolyte film having thickness of 50 microns is 25% or less based on JIS K 7136. The conversion haze value (H50) of the electrolyte film having thickness excluding 50 microns is 25% or less. The conversion haze value (H50) of electrolyte film having thickness of 50 microns is calculated according to the formula: H50=100.(1-((100-Ht)/100)50/t), where t is the film thickness (microns) and Ht is haze value of electrolyte film measured according to JIS K 7136. The total area of island particle of sea/island structure is 0.1-70% of the area of the electrolyte film. The density of island particles is 0.1-100 per 1 microns2 of the area of electrolyte film.Preferred Polymer: The basic polymer is nitrogen-containing aromatic basic polymer, preferably poly((2,2'-(m-phenylene)-5,5'-dibenzo imidazole).Preferred Process: The polymeric electrolyte film obtained after removal of fluid medium is preferably heat-processed. EXAMPLE - 5 mass% Nafion solution containing Nafion, water and isopropanol was prepared as a solution (C). The obtained solution (C) was mixed with dimethylacetamide (DMAC), refluxed at 120degreesC for 1 hour, and concentrated under reduced pressure, to obtain a solution (B) containing Nafion and DMAC in a mass ratio of 1.5/98.5. DMAC and poly(2,2'-(m-phenylene)-5,5'-dibenzimidazole) (PBI) having weight average molecular weight of 27000, were added to an autoclave. The autoclave was sealed, raised to 200degreesC, maintained for 5 hours, and naturally cooled, to obtain PBI solution containing PBI/DMAC in a mass ratio of 10/90. The PBI solution having intrinsic viscosity of 0.8 dl/g was diluted 10 times by DMAC to obtain a solution (A) having PBI/DMAC in mass ratio of 1/99. The obtained solution (B) (in g) (100), solution (A) (16.3) and solution (C) (97.1) were mix-stirred, and evaporated under reduced pressure at 80degreesC to obtain a cast liquid. The obtained cast liquid had Nafion (5.6 mass%) and PBI (0.14 mass%). The cast liquid (32) was poured into Petri dish, and heat-processed at 80degreesC for 1 hour. The solvent was removed, and the Petri dish was heat-processed in an oven at 160degreesC for 1 hour. The Petri dish was removed from the oven, and ion exchange water was poured into the cooled Petri dish. The peeled film was dried to obtain a polymeric electrolyte film. The obtained polymeric electrolyte film had high transparency and haze value of 0.6%. The obtained electrolyte film had sea/island structure containing island particles in a monopoly area ratio of 0.9%. The obtained film was used for polymer electrolyte fuel cell. The obtained fuel cell had high durability and output characteristics.