PURPOSE: To reduce heat generated, and to increase transmission force by operating hydraulic clutch arranged between the inner sleeve of a first rotary shaft an outer sleeve of a second rotary shaft by means of a pair of pistons so as to form a hydraulic pump to be operated through a difference between rotations of both rotary shafts and communicating the port of the hydraulic pump to a piston driving cylinder.

CONSTITUTION: When the end part 2 of an input rotary shaft rotates, an inner sleeve 14 rotates, and pump gears 26 and 28 are rotates relatively to an inner sleeve 14 by means of a ring gear 30 and pinions 32, 34 arranged on the end of the inner sleeve 14. Consequently, pressure difference is produced between parts 36 and 38. Accordingly, one of pistons 22 and 24 advances, and inner and outer plate holders 18 and 20 are pressed, and the inner sleeve 14 is connected to an outer sleeve 8 for transmitting turning torque to the end part 4 of an output rotary shaft. A certain amount of hydraulic oil leaks from a high pressure side between the gears and a casing to a low pressure side. When the relative rotational speed between the input and output shafts is low, the hydraulic oil leaks slowly, and the clutch is not engaged. When the relative rotational speed is high, the clutch is engaged. Consequently, output rotational speed can be controlled.