PURPOSE: To obtain a cycloolefin copolymer comprising polymer units from a cycloolefin and other polymer units in a specified ratio, being excellent in transparency, adhesiveness, dimensional stability, hardness, and hydrolysis resistance and being useful for coating materials, etc.

CONSTITUTION: There is provided the objective cycloolefin copolymer comprising 0.1-99.99 wt.%, based on the total amount of the cycloolefin copolymer, (A) at least one polymer unit of a cycloolefin such as a compound of formula I or II (wherein R1 to R4 are each H, a 1-20C hydrocarbon group or they may be suitably bonded to each other to form a ring or rings; and n is 2-10), 0-80 wt.% (B) polymer units of an acyclic olefin such as a 2-20C α-olefin, and 0.01-50 wt.% (C) peroxy oxygen directly bonded to carbon atoms of components A and/or B. This copolymer is obtained by reacting a cycloolefin copolymer containing components A to C with at least one derivatizing agent such as a reducing agent, an oxidizing agent, an esterifying agent, an etherifying agent, or a compound containing an α,β-unsaturated hydrocarbon.