To provide a resin composition having high resolution optical properties and keeping the properties for a long time, to provide a molding molded by using the resin composition, to provide a plastic optical element, and to provide an optical pickup device applied with the plastic optical element.

The resin composition comprises an alicyclic hydrocarbon-based copolymer represented by general formula (1), wherein the alicyclic hydrocarbon-based copolymer has ≤0.05 ratio B/A of absorbance (A) appearing at 2970±10 cm-1 to absorbance (B) appearing at 1720±10 cm-1, in infrared absorption measurement.