PURPOSE: To improve balance by a simple and small sized structure by a method wherein vanes are inserted into grooves passing through the center of a rotor, vanes are integrated mutually and are rotated in a cam ring whose diagonal length passing through the center of rotation becomes constant at all times.

CONSTITUTION: The vanes 1, 2, sliding in the grooves 3a, 3b passing the center O of the rotor 3, are connected mutually integrally by lapping the notches thereof or by utilizing a pin. The shape of the inner periphery 4a of the cam ring 4 is a circle described about a center point C apart from the trace of the middle point A of the vane by a length (r) from the center O of the rotor with a radius (r) and the point A rotates on the periphery with a rotating speed which is two times of the rotating speed of the rotor. Accordingly, a central point (c) of the center of gravity A of the vane and the center of gravity D of the vane 2 comes to a constant position at all times and rotating eccentric vibration will never be caused, therefore, the balance becomes good.