To provide a method for producing an electrolyte material that is excellent in oxidation resistance.

The method comprises a process for introducing a polymerization starting point into a base material, a process of grafting a vinyl monomer containing a monomer represented by formula (1) (where X is a halogen) to the polymerization starting point, a process of reacting an antioxidant having a functional group selected from an OH group, an amino group, and an imino group in the molecular structure through the functional groups, and a post-treatment process including a step of reacting a sulfonation agent. According to the method, the polymer material is formed as the base backbone of an electrolyte material before the polymer chain is coupled with the antioxidant, allowing the introduction of the polymer chain with the antioxidant without inhibiting the chemical reaction for forming the polymer chain, and further facilitating the polymer chain to couple with the antioxidant so strongly that the electrolyte material does not easily uncouple and can keep improved resistance to oxidation for a long time.