PURPOSE: To increase a life of a piston shaft suppressing local wearing by forming a sliding part, provided in the piston shaft, to come into contact with a bearing cylinder of a casing, in the case of a vacuum pump or the like.

CONSTITUTION: Adjacent to an operating chamber 5, a front part bearing cylinder 15 is extended to the rear, and a front part of a piston shaft 6 is guided by an internal surface of this bearing cylinder. In the periphery of the piston shaft 6, a rotary sliding cylinder 20 is externally fitted, and this sliding cylinder forms in its before and after a front sliding part 21 of large contour and a rear sliding part 22 of a little smaller contour. Ball bearings 23, 24 are provided between both the sliding parts 21, 22 and the piston shaft 6, and the rotary sliding cylinder 20 can be rotated about the piston shaft 6 by these bearings.