A polymer electrolyte membrane comprising as a main ingredient a block copolymer which comprises, as its constituents, a polymer block (A) having as a main unit an aromatic vinyl compound unit and a polymer block (B) forming a flexible phase, and has ion-conducting groups on the polymer block (A), said aromatic vinyl compound unit being such that the hydrogen atom bonded to the α-carbon atom is non-replaced or replaced with an alkyl group or an aryl group optionally having substituent(s), and at least one of hydrogen atoms directly bonded to the aromatic ring is replaced with an alkyl group; and a membrane electrode assembly and a polymer electrolyte fuel cell both of which uses it. The polymer block (A) can have a restraining phase, and/or can be cross-linked. The electrolyte membrane is mild to the environment, has a high ion conductivity and good bonding properties to electrodes, is excellent in moldability, and is not easily influenced by methanol; and displays excellent performance in polymer electrolyte fuel cells, particularly direct methanol fuel cells.