A novel poly(arylene ether sulfone) having a sulfoalkoxy group, which has both of heat resistance and electroconductivity, is suitable as polymer electrolyte membranes used for high performance polymer electrolyte fuel cells, and does not use fluorine so that loads to the environment are small in the synthesis or disposal, a process of producing the same, and a polymer electrolyte membrane comprising the same. The poly(arylene ether sulfone) having a sulfoalkoxy group has a structural unit represented by the formula (1) mentioned hereinabove, and is produced by reacting a poly(arylene ether sulfone) having a hydroxyl group with a specific alkali metal compound and/or an organic base compound, followed by reacting with a specific sulfonating agent and then with a specific acid. A polymer electrolyte membrane is produced by film formation of the polymer.