To provide a novel diisopropyl fumarate-cinnamic acid derivative-based copolymer expected to be formed into a retardation film having excellent optical properties ensuring that, for example, the refractive index in the film thickness direction and the out-of-plane retardation are large and even in the form of a thin film, the film has a high out-of-plane retardation, and a retardation film using the same. The diisopropyl fumarate-cinnamic acid derivative-based copolymer contains: a diisopropyl fumarate residue unit; and a cinnamic acid residue unit or a cinnamic acid ester residue unit having an alkyl group with a carbon number of 1 to 6.