To provide an olefinic resin having excellent heat resistance, transparency, nonhygroscopicity and chemical resistance with excellent casting workability, and to provide a copolymerization method for the resin.

The cyclic conjugated diene copolymer has (A) a molecular unit derived from a cyclic conjugate diene monomer and (B) a second molecular unit derived from an α-substituted vinyl aromatic monomer, and has a random structure. An another copolymer is obtained by hydrogenation or epoxidation of the above copolymer. An another method for producing the above copolymer comprises copolymerization between the cyclic conjugated diene monomer and the α-substituted vinyl aromatic monomer by living anionic copolymerization process.