To effectively remove tetrahydrodicyclopentadiene, by employing a high-boiling-point medium from the beginning of steps of addition-polymerizing an α-olefin and dicyclopentadiene in a medium, hydrogenating the copolymeric soln. thus formed, and then removing tetrahydrodicyclopentadiene formed through the hydrogenation, or by adding a high-boiling-point medium at the later stage.

The high-boiling-point medium for use at least contains a hydrocarbon medium having a b.p. of 195-300°C at normal pressure and an ignition point of ≥260°C. A preferable α-olefin is ethylene or propylene, in view of the polymerization activities and mol.wts. of the polymers. Propylene is esp. preferable. As a cyclic olefin, dicyclopentadiene is employed. This way, it is possible to remove, effectively and safely, tetrahydrodicyclopentadiene having a low ignition temp., which is formed as a byproduct of the hydrogenation, or prevent its formation. This copolymer is suitable as a material for optical use since it is highly pure, substantially without tetrahydrodicyclopentadiene.