Polyarylene block copolymer having sulfonic acid group and use thereof

A polyarylene copolymer having a sulfonic acid group which has high proton conductivity and reduced swelling in hot water and reduced shrinkage in drying; a solid polymer electrolyte and a proton conductive membrane comprising the copolymer; and a membrane-electrode assembly using these. The polyarylene block copolymer comprises a polymer segment (A) having a sulfonic acid group, and a polymer segment (B) having substantially no sulfonic acid group, the polymer segment (B) having substantially no sulfonic acid group comprising a structural unit represented by the following formula (1).