Abstract: A melt mixer comprises a vessel equipped with heating means, and a pair or more of rotor means including rotors polygonal in cross section and held within the vessel, the rotor means having a heating function. The rotors of each rotor means are angularly staggered to the rotors of the adjacent rotor means. Each rotor means comprises a plurality of rotors axially mounted on a rotating shaft, the rotors being angularly staggered and spaced in succession from one another. Also, the rotor means are so disposed as to be axially movable together with one jacketed side plate of the vessel.