PURPOSE: To prevent harmful stress from being caused in an elastic member for placing a vane in pressure contact with a rotor, by making the elastic member of a helical torsion spring, and layer the axis of the spring nearly in parallel with the axis of gyration of the rotor.

CONSTITUTION: A helical torsion spring 11 is located at one end portion 11a in contact with the back of a vane 8 and at the other end portion 11b with the bottom of the vane sliding groove 4a of a cylinder. The axis of the spring 11 is nearly parallel with that of a rotary shaft 6. The spring 11 acts in conjunction with the sliding of the vane 8. Since the force acting to the end portion 11a of the spring 11 produces twisting moment about the axis of the spring, almost only bending stress in caused in the wire constituting the spring. Furthermore, the end portion 11b of the spring 11 is rotatable on the bottom of the sliding groove 4a. As a result, no harmful stress is caused in the spring.