To reduce the number of assembling process and a cost by exhausing air to be mixed into working fluid on the bottom of a piston to outside of the piston quickly and by reducing the number of parts to be assembled inside of the piston.

Inside protruding parts 40b which are protruding inside of a stop ring almost in U-letter shape are formed at 3 positions on peripheral side corresponding to an air exhause port 23e in a stop ring 40 to be attached on a ring groove 23g of a piston 23 and a concave part for air exhaust 40c is formed outside of each inside protruding part 40b. The concave part for air exhaust 40c is formed in a depth in which a bottom wall is positioned inside of the stop ring 40 rather than the inside circumference of a cylindrical part 23b, and an air vent hole 41 is made with the concave part for air exhaust 40c and the air exhaust port 23e of the piston 23. One end of an adjust spring is directly hung on the inside protruding parts 40b.