The present invention relates to a method for preparing a cyclic olefin polymer by addition polymerization, and more particularly to a method for preparing a norbornene-based addition polymer, which comprises a step of contacting a norbornene-based monomer having an ester or acetyl group with a catalyst system comprising: a Group X transition metal compound; a compound comprising a neutral Group XV electron donor ligand having a cone angle of at least 160.degree.; and a salt capable of offering an anion that can be weakly coordinated to the transition metal, in solvent. The norbornene-based addition polymer having an ester or acetyl group according to the present invention is transparent, has superior adhesivity and a low dielectric constant, and it generates no byproducts when attached to metal. Therefore, it can be used as an optical film, a retardation film, a protection film for a polarizer, POFs (plastic optical fiber), PCBs (printed circuit board), or insulators of electronic devices.