PURPOSE: To carry out the manipulation of a clutch only by gripping and ungripping a switch together with a knob of a shift lever when the driver manipulates the shift lever, to also use engine brake and further to restrain abrasion.

CONSTITUTION: A front receiving disc 7 and a rear receiving disc 4 are assembled together. When a main shaft gear is meshed with a counter shaft gear in a gear casing 10 which fluid is circulated, and is rotated, if the flow of the fluid which repeats circulation from the high pressure side to the low pressure side, is stopped by a solenoid valve, only the rotation of the counter shaft gear is stopped by a pressure of the fluid corresponding to a load, and accordingly, the rotating motion is changed by the effect of the counter shaft, about the main shaft 8 as the center.