A process of producing a ring-opening metathesis polymer characterized in that a treatment to decrease the amount of oxygen and/or peroxide in at least one kind of polymerization starting material is performed prior to the ring-opening metathesis polymerization reaction of a cyclic olefin performed in the presence of a ruthenium carbene complex (catalyst). Treatments to decrease the amount of oxygen and/or peroxide include applying an adsorbent to a polymerization starting material to remove oxygen and/or peroxide by adsorption, applying an antioxidant to a polymerization starting material to decompose oxygen or peroxide, and the like.