Novel polymer electrolyte, polymer electrolyte composition, electrolyte membrane, and production method and use thereof

A polymer electrolyte comprising a hydrophilic segment having sulfonic acid groups and a hydrophobic segment having no sulfonic acid group, each segment having an aromatic ring in its main chain, wherein the polymer electrolyte is an aromatic block copolymer having an ion-exchange capacity of solely the hydrophilic segment at 3.6 mmol/g or more, and an ion-exchange capacity of the block copolymer in the range of from 0.5 mmol/g to 3.0 mmol/g; a polymer electrolyte membrane produced from the polymer electrolyte and a fuel cell using thereof.