The norbornene derivative of the present invention is represented by the following formula (Im). ##STR00001## According to the present invention, a novel norbornene derivative that is useful as a precursor monomer for preparing a cycloolefin polymer exhibiting excellent transparency, heat resistance and low-water absorption properties and having been freely controlled in the birefringence properties and the wavelength dispersion properties can be provided. According to the present invention, further, a norbornene ring-opened (co)polymer having excellent transparency and heat resistance, exhibiting high solubility in organic solvents and having specific birefringence properties and wavelength dependence, and a process for preparing the ring-opened (co)polymer can be provided.