PURPOSE: To regulate the rear end position of a piston stroke so as to suppress the occurrence of a failure by allowing a cushion chamber to be partitioned between the rear end face of a piston trying to continue its retreat by inertia and the valve plug of a rear oil chamber so as to confine pressure oil thereto.

CONSTITUTION: A piston 1 having small diameter parts 1A, 1C at the front and rear parts and a large diameter part 1B at the intermediate part is fittingly mounted into a cylinder 2 to partition a front and a rear oil chambers. A valve chamber 7 is partitioned between the outer periphery of a valve plug 5 fittingly secured to the cylinder 2 and the inner periphery of the cylinder 2, and a cylindrical change-over valve 6 is accommodated in this valve chamber 7. The piston 1 is reciprocated to obtain stroke action by switching the oil pressure of the rear oil chamber to high/low pressure by the change-over valve 6. In this case, the rear inner diameter of the valve plug 5 is formed into the approximately same diameter as the large diameter part 1B of the piston 1, and the rear end part inner diameter of the valve plug 5 is formed into the approximately same diameter as the small diameter part 1C. A cushion chamber 27 to which pressure oil is confined by the retreat of the piston 1 can be partitioned so as to regulate the rear end position of a piston stroke by the confinement of pressure oil.