A crystal of carboxyl acid and at least one compound selected from the group consisting of amine and ammonia are mixed in an absence of a liquid medium. The carboxyl acid is conjugated diene such as (Z,Z)-muconic acid, the conjugated diene including a carboxyl group. The at least one nitrogen-containing compound selected from the group consisting of amine and ammonia is primary amine represented by RCH2NH2, where R is an aryl group, or is a normal alkyl group that contains five or more carbon atoms. Moreover, such amine that is a bivalent group and that is represented by —Ar—N═N—Ar′—, where Ar and Ar′ are independently aromatic hydrocarbon bivalent groups, is intercalated in a layered crystal of a polymer such as polymuconic acid, the layered crystal including a carboxyl group. In this way, a novel photoresponsive polymer is obtained.