To obtain a cyclic olefin polymerization catalyst capable of realizing a high cyclic olefin conversion in an extremely small amount, and to provided 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) water or an acid. A method for producing the cyclic olefinic polymer comprises polymerizing a cyclic olefin in the presence of the catalyst. This method for producing the cyclic olefinic polymer comprises polymerizing the cyclic olefin in the presence of a catalyst comprising (A) the transition metal compound and (B) the compound capable of reacting with the transition metal component to form the ionic complex in water as a dispersing medium.