PURPOSE: To arrange so that the captioned masticated rubber may be secured by discharging masticated rubber when mastication operation is terminated simultaneous with the drive torque of the rotor of a kneading machine reaching a prearranged value during the mastication operation of natural rubber on closed-type kneading machine.

CONSTITUTION: When raw material natural rubber is kneaded by closed-type kneading machine such as Banbury type mixer, etc., the drive torque value of the rotor of a kneading machine proportional to the plasticity of masticated rubber is measured and mastication operation is terminated when the drive torque value reaches a prearranged value. Then masticated rubber is discharged from closed-type kneading machine, thus enabling the availability of masticated rubber having constant plasticity. Measurement of the drive turque of the rotor is performed other than direct means by torque meter, wattmeter measuring method wherein input for rotor-driving electric motor is measured by wattmeter.