A method of producing an electrolyte membrane for use in a fuel cell, including: performing radiation-induced graft polymerization of a vinyl monomer having a nucleophilic functional group selected from an acylvinyl ether derivative, a styrene derivative, and a methacrylic acid derivative, with a polymer substrate having a fluorine-containing polymer, an olefin-containing polymer, or an aromatic polymer; deprotecting 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 nucleophilic functional group of the graft chain thus deprotected, by use of an electrophilic reagent selected from cyclic sulfonic acid ester and alkylhalide sulfonate.