There is provided a ring-opening polymer of cyclopentene wherein a cis ratio of the cyclopentene-derived structural units is 30% or more, a weight average molecular weight (Mw) is 100,000 to 1,000,000, and a functional group containing a structure represented by the general formula (1) or (2) is included at an end of the polymer chain.
—Y—H (1)
(in the above general formula (1), Y represents an oxygen atom or a sulfur atom.)
—NH-Q (2)
(in the above general formula (2), Q represents a hydrogen atom or a hydrocarbon group or a silyl group which may have a substituent group.)