The present invention provides a method for preparing homo- and co-polymers by polymerizing cyclic olefin compounds using addition polymerization. The method can produce a cyclic olefin polymer having a high molecular weight with a high yield in the presence of, as an addition polymerization catalyst, a complex prepared by mixing a nickel salt compound and an organoaluminoxane compound with at least one fluorine-containing aromatic hydrocarbon compound represented by the following formula 1 or 2 as a catalyst activator:

C₆R_mH_6-mFormula 1

• where R is CF₃; and m is from 1 to 3, or

  C₆F_nH_6-nFormula 2

• where n is from 1 to 6.