This invention relates to a polymer electrolyte membrane for polymer electrolyte fuel cells, comprising a block copolymer which comprises, as its constituents, a polymer block (A) having as a main unit an aromatic vinyl compound unit whose α-carbon is quaternary carbon, and a flexible polymer block (B), and has ion-conducting groups on the polymer block (A).The electrolyte membrane of this invention is economical, mild to the environment and excellent in moldability and chemical stability and thus durability, and, therefore, can suitably be used in membrane electrode assemblies and polymer electrolyte fuel cells.