The present invention provides a cyclic olefin addition copolymer which has a reactive silyl group having a specific structure, excels in optical transparency, heat resistance, and adhesion, and is capable of producing a crosslinked product having improved dimensional stability, solvent resistance, and chemical resistance. The present invention also provides a process for producing the cyclic olefin addition copolymer, a crosslinking composition, a crosslinked product and a process for producing the same, and an optically transparent material (transparent resin film) comprising the cyclic olefin addition copolymer. The optically transparent material excels in optical transparency and heat resistance, exhibits improved dimensional stability, adhesion, solvent resistance, and chemical resistance, and is capable of improving fragility and preventing occurrence of cracks in the film.