To easily install a dustproof mechanism without restricting use by constituting a hollow member composed of a porous and freely extensible- contractible gas permeable material so as to elongate in the circumferential direction as well as to be compressed in the shaft direction even in a condition where a piston rod advances most.

A hollow member 10 composed of sponge is fitted around a piston rod 2, and is pressed against an end surface of a cylinder sleeve 1 by a head part 12 of the piston rod 2 through an annular presser plate 11 similarly fitted around the piston rod 2. In a natural condition before installation, an inside diameter of the hollow member 10 is set smaller than an outside diameter of the piston rod 2, and its shaft directional length is set larger than a height of a member housing space when the piston rod 2 advances most from the cylinder sleeve 1. When the hollow member 10 is installed, the screwing-in type head part 12 is removed, and presser plate 11 is removed. Since the hollow member 10 easily extends-contracts in the circumferential direction and the shaft direction, its installing work is facilitated.