A shearing action of rubber by a rotor wing has a relation to a wing perpendicular component P.sub.z of P (see FIG. 4), and the larger the value of P.sub.z, the larger the shearing action. An agitating action of rubber has a relation to a wing parallel component P.sub.x of P (see FIG. 4), and the larger the value of P.sub.x, the larger the agitating action. In order to maintain the shearing action and the agitating action of rubber at a high level, in an internal rubber mixer a helix angle of a long wing 2 is set in a range of not less than 30.degree. and less than 50.degree.. Further, an axial directional length L.sub.s of a short wing 3 is made larger than that (.delta.) of a cut-off portion of the long wing 2. A rubber flow f-1 is thereby prevented from passing straight through a rotor space portion. In this way, the internal rubber mixer is excellent in the shearing action of rubber, whereby enhances productivity and micro-dispersibility of a compounding agent, and is excellent in the agitating action of rubber, whereby it enhances macro-dispersibility of the compounding agent and low temperature mixing of rubber.