To obtain a catalyst enabling to obtain a high mol.wt. cyclic olefin polymer in high activity, and to provide a method for producing the cyclic olefin polymer.

This catalyst for polymerizing a cyclic olefin comprises (A) an organic transition metal compound and (B) an organic aluminum oxy compound. Therein, the organic transition metal compound A is represented by the formula: M(H2O)nAxByCz wherein A is a chelating bidentate ligand containing at least one kind of element selected from nitrogen, oxygen and phosphorus; B expresses a σ-binding ligand; C is a Lewis basic ligand; (x) is an integer of 0-2; (y) and (x) are each an integer of 0-4; when (x) is 2, (y) and (z) are each 0; when (x) is 1, (y) and (z) are each 2; when (x) is 0, (y) and (z) are each 4; (n) is integer of 1 or 2; M expresses the group VIII transition metal in the periodic table. This method for producing the cyclic olefin polymer comprises bringing a cyclic olefin such as norbornene into contact with the catalyst.