A process for producing a modified polymer rubber comprising the steps of: (1) polymerizing a conjugated diene monomer or a combination thereof with an aromatic vinyl monomer in a hydrocarbon solvent, in the presence of an alkali metal catalyst or a specified alkali metal compound, to form an alkali metal end-carrying active polymer, and (2) reacting the alkali metal end-carrying active polymer with a specified isocyanurate compound or a combination thereof with a specified coupling agent in a hydrocarbon solvent.