PURPOSE: To suppress the generation of vibration due to low pressure air by spacing between openings by a rate a little more than the thickness of a piston, and thereby communicating a first chamber with a second chamber by means of a bypass when the piston is moved between its upper and lower positions.

CONSTITUTION: A bypass A is opened when a valve 28 is at its first position, and closed when the valve 28 is at its second position. The bypass A is operated only while a piston 10 is moved downward from a first position to a second position. A bypass B has a first opening 36 which is positioned near a lower surface of the piston 10 when the piston is at the first position, and a second opening 38 spaced therefrom by a rate a little more than the thickness of the piston 10. The bypass B, therefore, connects both sides of the piston 10. The bypass A speed-up the downgoing motion of the piston, while the bypass B reduces angular vibration at the ending of the upgoing motion of the piston.