A process for manufacturing a cycloolefin addition polymer includes polymerizing monomers containing a cycloolefin compound using a catalyst containing a nickel compound or a palladium compound by addition polymerization in the presence of a molecular weight controlling agent in two steps, that is a step of a initiating the polymerization reaction using the monomers in an amount of not more than 80 wt % of the total monomers and a step of supplying the remaining monomers to the reaction system during the polymerization reaction. A cycloolefin addition polymer with a uniform quality, having a narrow molecular weight distribution and a controlled molecular weight, and excellently balanced processability and mechanical strength can be obtained at a high polymerization conversion rate using the process. The process can be operated at a highly controlled polymerization temperature, and is thus suitable for industrially manufacturing a cycloolefin addition polymer.