PURPOSE: To efficiently transfer the engine power to an output shaft at the time of high speed operation by placing a hydraulic pump between an input shaft and an output shaft and arranging a valve member in a circulating- working-oil-passage for the hydraulic pump together with interposing a clutch mechanism between the input shaft and the output shaft.

CONSTITUTION: Until the revolving speed of an input shaft 4 reaches the predetermined speed, the captioned clutch functions as an ordinary, statical fluid pressure coupling, in which the revolving speed of the output shaft 7 is smaller than that of the input shaft, owing to the effect of a gear pump 5, thereby effectively preventing abnormal vibration or noise during the low engine speed. When the input shaft revolving speed exceeds the predetermined speed, the working oil, which circulates in a passage 24 for working oil circulation, is interrupted by a spool 16 for a centrifugal valve. As a result, the operation of a gear pump 5 is suspended, and both output shaft 7 and input shaft 4 enter a synchronized revolution. With this contrivance, a clutch mechanism 29, which is in a uncoupled state, can smoothly be coupled. Therefore, the output shaft 7 can easily be locked up against the input shaft 4.