To keep function even when pressure medium escapes by providing a pressing body which is housed in a rotary cylinder and presses an outer peripheral surface of a rotary shaft through pressing force of a pressing member and a force functioning part which applies force against pressing force to the pressing body.

When high pressure oil is supplied to an oil supply passage 43 of a driven side fitting yoke 15, a piston part of a piston device 24 is recessed into the yoke 15 against a plate spring. A flat surface 21a of a roll shaft 17 is opposed to a flat surface 23a of a fitting hole 22. The roll shaft 17 is inserted into the fitting hole 22, for stopping supply of high pressure oil. When the piston part of the piston device 24 is projected into the fitting hole 22 by means of the plate spring, a side which is going to abut against the flat surface 23a is pressed at the driving time of the roll shaft 17. As a result, the roll shaft 17 is fixed to the yoke 15 through the piston device 24, for preventing occurrence of vibration. Supply of operation oil is not required at the time of driving, so that torque transmission function is not eliminated even when operation oil escapes at the time of driving.