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. Also obtained is a crystal of ammonium carboxylate including at least one kind of carboxyl acid and at least one kind of amine, at least one kind of compound selected from (i) the at least one kind of carboxyl acid and (ii) the at least one kind of amine being a diacetylene derivative. The crystal of ammonium carboxylate is subjected to light or heat, to obtain a built-up type diacetylene polymer.