The reinforcement of rubber filled with carbon black is performed through three steps change of the state of rubber molecule. Firstly, rubber molecules are absorbed very strongly on the surface of carbon black in mixing as bound rubber and their uncross-linked states are kept as they were even after vulcanization. Second, with increasing the black content, adjacent uncross-linked bound rubbers adhere with each other and form the connected continuous networks. Third, under large extension the rubber molecules within the bound rubber are extended, orientated and finally changed to a bundle of ultimately-extended molecules. The bundles thus formed co-continuously in the system compensate for the weakness of the cross-linked rubber networks, resulting in the generation of the stress-upturn and high tensile strength for rubbers.