There is provided a ring-opening polymer of cyclopentene wherein a cis ratio of cyclopentene-derived structural units is 30% or more, the weight average molecular weight (Mw) is 100,000 to 1,000,000, and an oxysilyl group is included at an end of the polymer chain. For example, the ring-opening polymer of cyclopentene can be obtained by ring opening polymerization of cyclopentene in the presence of a compound of a transition metal belonging to Group 6 in the Periodic Table, an organoaluminum compound represented by the following general formula (1), and an olefinically unsaturated hydrocarbon containing an oxysilyl group. (R1)3-xAl(OR2)x  (1) (in the general formula (1), R1 and R2 represent a hydrocarbon group having 1 to 20 carbon atoms and x satisfies the requirement 0