PURPOSE: To reduce loads applied on components by fitting a columnar rotor, which has a plurality of spiral grooves reducing the pitch from one end toward the other gradually wherein spiral blades are fitted in these grooves, eccentrically in a cylinder, and thereby obtaining equilibrium of the thrust force.

CONSTITUTION: A stator 5 of an electromotive element 3 is fixed to the inner surface of an enclosed case 2, and a cylinder 7A is fitted on a rotor 6 arranged inside thereof, wherein the two ends are supported rotatably by bearings 8, 9, which are provided with suction holes 21, 22. A piston 10 is fitted in this cylinder 7 as a rotating element eccentrically in an amount (e). This piston 10 is provided at its periphery with two spiral grooves 13, 14 reducing the pitches gradually from both ends toward the middle part in axial direction, and spiral blades 15, 16 are fitted in these grooves 13, 14 in such a way that they can advance and retreat freely. The rotor 6 of the motor-driven element 3 is equipped with a discharge path 27 leading to a discharge hole 28, which is provided in the middle of cylinder 7 in its axial direction.