To obtain a cyclic olefin polymerization catalyst capable of realizing a high cyclic olefin conversion in an extremely small amount, and to provide a method for producing a high mol.wt. cyclic olefin polymer.

This catalyst for polymerizing a cyclic olefin comprises (A) a transition metal compound, (B) a compound capable of reacting with the transition metal compound to form an ionic complex, and (C) a compound selected from the group consisting of an alcohol, an aldehyde and a ketone. A method for producing a cyclic olefinic polymer comprises polymerizing the cyclic olein in the presence of the catalyst.