To prevent damage of the cylinder inside face resulting from shocks or the like, by providing a skirt-shaped part protruding from the front end edge of the slide face contacting the outer periphery of the front end to the front end direction while gradually separating from the cylinder inside face and integrally extending a curved face constituting at least a part of the front end face.

In a skirt-shaped part 4, a curved face 5 protruding from the front end edge of the slide face 3 to the front end while gradually separating from the inside face of the cylinder 2, is integrally extended. The skirt-shaped part 4 has an upward skirt-shaped part 4a or a downward skirt-shaped part 4b. In the case of the upward skirt-shaped part 4a, the upper end face is formed as a semicircle protruding upward the section and the external half side is formed as a curved face 5 protruding to the front end while gradually separating from the cylinder 2 inside face. In the case of the downward skirt-shaped part, the lower end face is formed as a semicircle protruding downward the section and the external half side is formed as a curved face 5 protruding to the front end while gradually separating from the cylinder 2 inside face.