A polymer electrolyte satisfying both of proton conductivity and chemical stability such as water resistance at a high level that is preferable as the polymer electrolyte for fuel cells and the like is provided. The invention includes a block copolymer comprising one or more segments having an ion exchange group and one or more segments having substantially no ion exchange group, wherein at least one of the segments having an ion exchange groupis the segment represented by the following general formula (1A), (1B) or (1C): and the segment has ion exchange group density of 4.0 meq/g or more, and the segments having substantially no ion exchange groupis the segment represented by the following general formula (2):