An electrolyte membrane having alkylether graft chains for use in a fuel cell produced by a method of producing an electrolyte membrane for use in a fuel cell, including: performing radiation-induced graft polymerization of a vinyl monomer having nucleophilic functional groups, the vinyl monomer selected from an acylvinyl ether derivative, a styrene derivative, and a methacrylic acid derivative, with a polymer substrate comprising a polymer selected from a fluorine-containing polymer, an olefinic polymer, and an aromatic polymer; deprotecting the nucleophilic functional group, which is protected by an ester bond, of a graft chain on the polymer substrate introduced by the radiation-induced graft polymerization; and introducing an alkylethersulfonic acid structure into the deprotected nucleophilic functional group of the graft chain, by use of an electrophilic reagent selected from cyclic sulfonic acid ester and alkylhalide-sulfonate.