NOVELTY - A polymer electrolyte membrane (12) contains polyelectrolyte having ion exchange group. The ratio of sum (Sp) of peak area of the membrane after immersing in aqueous ferrous chloride tetrahydrate solution at 25 degrees C for 1 hour and drying at 25 degrees C and 10 hPa or less for 12 hours, with respect to sum (Snp) of peak area of the membrane before immersing in water at 25 degrees C for 1 hour and drying at 25 degrees C and 10 hPa or less for 12 hours, obtained by 13C nuclear magnetic resonance spectrum, is 0.42 or less.    USE - Polymer electrolyte membrane used in membrane-electrode assembly of fuel cells (both claimed) used in electric equipment and automobile industry.    ADVANTAGE - The polymer electrolyte membrane has high radical tolerance, high proton conductivity and water resistance, and provides fuel cells having high electric power generation efficiency.    DETAILED DESCRIPTION - INDEPENDENT CLAIMS are included for the following:    (1) polymer having ion exchange capacity of more than 3.0 meq/g, which consists of polyarylene structure in principal chain and sulfonic acid groups connected to portion(s) of aromatic ring in principal chain containing 1-20C alkyl having fluorine atom, 1-20C alkoxy group, 6-20C aryl group, 2-20C acyl or 6-20C aryloxy group (all optionally substituted);    (2) polyarylene type block copolymer, which contains block which does not have ion exchange group of formula: -(Ar2-X1)- (B-2) and block having ion exchange group of formula:-(Ar1)- (B-1), and has weight average molecular weight of 4000-25000 by polystyrene conversion;    (3) polyelectrolyte, which contains the polymer;    (4) polyelectrolyte composite film, which consists of polyelectrolyte provided on porous base material;    (5) catalyst composition, which contains polyelectrolyte and catalyst component;    (6) membrane-electrode assembly, which consists of polymer electrolyte membrane and catalyst layer (14a,14b) provided on the membrane; and    (7) fuel cell, which consists of a pair of gas diffusion layers (16a,16b) provided between a pair of separators (18a,18b), and membrane electrode assembly positioned at the gas diffusion interlayer.    Ar1=arylene optionally substituted by 1-20C alkyl, 1-20C alkoxy, 6-20C aryl, 2-20C acyl, 6-20C aryloxy (all optionally substituted), or cyano group;    Ar2=bivalent aromatic group optionally substituted by 1-20C alkyl, 1-20C alkoxy, 6-20C aryl, 2-20C acyl, 6-20C aryloxy (all optionally substituted), or cyano group;and    X1=oxygen or sulfur.    At least one ion exchange group is directly bonded through the aromatic ring in Ar1.    DESCRIPTION OF DRAWING(S) - The drawing shows the cross-sectional view of the fuel cell.    Fuel cell (10)    Polymer electrolyte membrane (12)    Catalyst layers (14a,14b)    Gas diffusion layers (16a,16b)    Separators (18a,18b)