A block copolymer comprising at least one segment having an acid group and at least one segment substantially free from acid groups is provided wherein the segment substantially free from acid groups contains a structure of the following general formula (1): Ar1—Z—Ar2—O—Ar3—Om  (1) (wherein, m represents an integer of 10 or more, Ar1, Ar2 and Ar3 represent mutually independently a divalent aromatic group which may be substituted by an alkyl group having 1 to 10 carbon atoms, an alkoxy group having 1 to 10 carbon atoms, an aryl group having 6 to 10 carbon atoms or an aryloxy group having 6 to 10 carbon atoms, and Z represents —CO— or —SO2—, each Z in the segments representing independently —CO— or —SO2—). The block copolymer exhibits more excellent performances as a polymer electrolyte for fuel cells or the like.