PURPOSE: To protect a clean environment from contamination by a method wherein the contact part of a piston rod and a cylinder body is covered with a protective cover and the one end of the protective cover is kept in non-contact condition with the movable part of the main part and, moreover, a positive pressure space into which clean air is supplied is provided between the end of the protective cover and the movable part.

CONSTITUTION: The sliding part between a cylinder body 2 and a stationary rod 4 is covered with a protective cover 3 and a positive pressure space 12 to receive clean air 20 is provided at the end part of the protective cover 3 to prevent sputtering of dust particles from the sliding part so as to provide a clean environment in which the cylinder 1 is installed. By making a space between the end of a partition wall 11 and the side surface of the cylinder body 2 wider than a space between an outer wall 10 and the side surface of the cylinder main part 2, more circulation of the clean air 20 which is supplied into the positive pressure space 12 can be provided in a circulation space 13. With this constitution, the dust particles produced by the sliding part in the circulation space 13 can be prevented more effectively from being sputtered out.