To obtain the subject palladium catalyst which shows improved activity in polymerization of cycloolefin, can extremely lower the weight ratio of the catalyst used to the amount of the produced polymer and can remain the catalyst in the polymer.

This palladium compound is represented by the formula (E is nitrogen, phosphorus, arsenic, antimony; X is O, NR1, S; R1-R3 are a 1-20C alkyl, a 2-30C alkyl, a 3-8C cycloalkyl; R4 and R5 are each H, a 1-20C alkyl, a 2-30C alkyenyl, a 3-8C cycloakyl; L is an alkyl ether or an ester). This compound is preferably prepared by using a cyclopalladium compound bearing readily substitutable ligands such as cyclooctadiene as a starting substance and the (co)polymerization of a cycloolefin is carried out in the presence of an acid cocatalyst at 0-200°C.