To obtain the subject polymer useful for a rubber composition such as a tire casing, etc., and specifying its trans bond content by polymerizing at least one kind of a diene monomer by means of a lithium catalyst system.

This method for producing an amino-functionalized diene by polymerizing at least one kind of a diene monomer with a lithium catalyst system, is provided by using the above catalyst system consisting of (A) at least one kind of a hydrocarbon solvent, (B) one kind of a compound of IIIA group metal, (C) one kind of a compound of an alkaline earth metal and (D) one kind of a lithium initiator. Further, it is preferable to prepare a co-catalyst by reacting the components B with C each other in the component A, making the above co-catalyst in contact with one or more kinds of diene monomers, reacting the obtained mixture with the component D, and then stopping the polymerization reaction to obtain the polymer functionalized with a chain terminal amino function.