PURPOSE: To perform power transmission without using frictional force of a disc by relatively revolving first and second gears of a gear pump at the time of an oil path opening operation of a clutch and preventing relative revolution by hydraulics on an exit side at the time of a closing operation.

CONSTITUTION: A hydraulic clutch has a pump mechanism 1, a valve mechanism 2 and an oil path opening and closing control mechanism 3. When revolving speed of a pump external housing 11 is increased by the driving of a power transmission means 4, each roller 300 tends to move to a two-dot chain line location by centrifugal force, and when revolving speed exceeds prescribed speed, a pulley 304 is pushed, a valve body 20 is moved in the right direction and an oil path closing location is made. Then, hydraulics is raised by oil delivered from a gear pump 10, a main driving gear 100 is not capable of racing around a driven gear 102 and is fixed. Therefore, a clutch is turned on. By the operation of a clutch lever 311, the clutch is turned off. Thus, power transmission is performed without using frictional force of a disc and wear is eliminated.