PURPOSE: To obtain a polymer having excellent optical suitability, by heat- treating a dicyclopentadiene alone or a mixture of dicyclopentadiene and a dienophile to give the title monomer and subjecting the monomer to ring opening polymerization.

CONSTITUTION: A dicyclopentadiene alone or a mixture of the dicyclopentadiene and a dienophile (aliphatic or aromatic olefin) is heat-treated in an inert gas atmosphere at 150W230°C for 1W10hours to give a tetra or more cyclic polycyclic norbornene monomer, solvent-insoluble components existing in the resulting substance are removed by filtration or solvent extraction to give the above-mentioned monomer having ≥85% light transmission, which is subjected to ring opening polymerization to give a polymer. The heat treatment is carried out preferably in the presence of a phenolic antioxidant. The polymerization is done in the presence of a catalytic system containing a transition metal compound and an organic metal or the transition metal compound, the organic metal and a third component such as triethylamine.