PURPOSE: To improve reliability of work by forming a supply chamber of a second sealing sleeve in a ring chamber, arranging it in a housing impossible to slide and as a stopper in the axial direction for a piston at a releasing position.

CONSTITUTION: Piston rod 5 and a piston 2 are moved to the left when a pedal 19 is actuated against energizing force of a spring 21. As a result, a first sealing sleeve 3 closes a pressure chamber 14 from a ring chamber 12 filled with a pressure medium after exceeding a position of a stopping hole 29. Consequently, pressure of the pressure chamber 14 increases, and this pressure is supplied to a slab cylinder through a pressure port 13. The piston 2 and the piston rod 5 are moved to an initial position on the right when clutch work is finished or a load to the pedal 19 is removed. At this time, the pressure medium in the pressure chamber 14 is lowered, and the piston 2 is again gently made contact with a second sealing sleeve through a stopping washer 8.