PURPOSE: To obtain a structure which will not be affected by manufacturing tolerance generated by an inversion processing of pressure housing.

CONSTITUTION: Both sides of a cylinder 2 are connected with orifices 7, 7a of closing members. These orifices 7, 7a have a diameter larger than that of the cylinder and cushion rings 8, 8b are mounted on them. Cushion pins 6, 6a are inserted into the cushion rings 8, 8b by a working piston 3 at each edge end. At least a cushion ring 8 for the position 'off' has a collar piece 8a, which positions the cushion ring 8 in the center to engage with the cylinder 2.