To provide a rubber composition which has fracture characteristics, abrasion resistance, and low heat build-up simultaneously kept at high levels, a copolymer used for preparing the same, and a production method thereof.

A modified diene polymer is produced by polymerizing or copolymerizing a conjugated diene monomer in a hydrocarbon solvent in the presence of an organolithium compound as an initiator and reacting the polymerization-active end groups of the resultant polymer or copolymer with a cyclic aminosilane compound represented by the formula. A rubber composition is prepared by using the modified diene polymer.