To provide such a method for manufacturing a ring opening methathesis polymer as gives a ring opening methathesis polymer of a cyclic olefin at a higher conversion by using, as a catalyst, a ruthenium carbene complex.

The method comprises a step, prior to the ring opening methathesis polymerization reaction of a cyclic olefin in the presence of a ruthenium carbene complex (a catalyst), of carrying out the treatment of reducing the amounts of oxygen and/or an peroxide in at least one kind of polymerization raw material. The polymerization raw materials include a variety of materials, which are used for the ring opening methathesis polymerization reaction and exist in the reaction system, such as the ruthenium carbene complex (the catalyst), the cyclic olefin (the monomer) and additionally a solvent and a chain transfer agent. The treating methods of reducing the amounts of oxygen and/or the peroxide include, for example, a method for adsorbing and removing oxygen and/or the peroxide by adding an adsorbent to the polymerization raw materials and a method for decomposing oxygen and the peroxide by adding an oxidation inhibitor to the polymerization raw materials.