PURPOSE: To make a controller have both torque control and clutch functions by normally opening an external discharge port of hydraulic fluid to the outer circumferential part of a housing body, and installing control part, altering an internal hydraulic fluid level from zero to the specified quantity, in a hydraulic fluid supply route.

CONSTITUTION: A hydraulic fluid supply route 23 is partitioned off between a boss cylinder 19 and an output shaft 22, and each of external discharge ports 25 and 26 of hydraulic fluid is formed in the outer circumferential part and side of a housing 16. The hydraulic fluid is fed to the passage 23 by way of a gear pump 30, a variable aperture 31 and a coupling 32, while each detection value of an exhaust temperature sensor 35 and a revolution pickup 36 is inputted into a controller 34, and the variable aperture 31 is driven via an actuator 33. Then, a hydraulic fluid quantity inside the housing is controlled from zero in a clutch-off state to the minimum requisite oil quantity during operation. Consequently, a torque transfer characteristic is made variable and, what is more, makes the controller have a clutch function jointly.