PURPOSE: To obtain a mixture of the subject isomers rich in exo isomer capable of further improving the heat-resistance and mechanical strength of a cycloolefin random copolymer by contacting an endo isomer of the subject compound with a solid acid catalyst, thereby isomerizing to exo isomer.

CONSTITUTION: The endo isomer II of a pentacyclo[4 . 7. 0.12. 5 . 08. 13 .19,12]pentadecene-3 expressed by formula I [R1 to Rl4 are H, hydrocarbon group or halogen; R11 (or R12) and R13 (or R14) may together form a monocyclic or polycyclic group] or a mixture of the endo isomer II and the exo isomer III and containing ≥85mol% of the endo isomer II is brought into contact with a solid acid catalyst to isomerize the endo isomer II into the exo isomer III to obtain an isomer mixture having a II/III molar ratio of 80/20 to 0/100. A cycloolefin random copolymer produced by the copolymerization of the mixture with ethylene has remarkably improved heat-resistance and mechanical strength.