PURPOSE: To increase durability at low cost by providing a valve means for varying the opening area of a flow resistance producing means in one rotating member and also providing a diaphragm mechanism operated pneumatically for controlling the valve means integrally together with the valve means.

CONSTITUTION: A needle valve 38(valve means) for varying the opening area of an orifice 28, a means to produce flow resistance, is stored free to move in a plug member 39 in which a rotor 1 is inserted, and a diaphragm mechanism 40 rotated together with the valve 38 as an unit and controlling the movement of the valve 38 is provided integrally at the rear end of the valve 38. When a negative pressure at the first port 49 with the second port 50 vent to the atmosphere, the negative pressure acts on the first chamber 42A through an A Chamber 48A and a connecting hole 41A, and the valve 38 is moved to the left by a diaphragm 43 to choke the orifice 28. Accordingly, a transmission torque is increased according to the negative pressure. As a result, the transmission torque can be controlled freely by varying negative pressure irrespective of a difference in rotating speed.