PURPOSE: To enhance polymerizing activity of the subject monomer so as to produce copolymer having high thermal stability by heating a coarse norbornane- based monomer containing a polymerizing inhibitor and a specific amount of dicyclopentadiene.

CONSTITUTION: A coarse morbornane-based monomer containing a polymerizing inhibitor, and about 10 wt.% of more dicyclopentadiene in all monomers are heated, and polymerization conversion ratio of an unthermally treated norbornane-based monomer is improved by 1% or more. The content of the polymerizing inhibitor is preferably in a quantity by which the monomer polymerization conversion ratio due to a heat balancing method is reduced at least by 1%.