A mounting concave portion with a predetermined depth is formed on a gas-pressure-receiving-side surface of a piston. Further, a plate-like sealing member includes a mounting convex portion which is erected at a position meeting with the mounting concave portion on the gas-pressure-receiving-side surface of the piston. When the piston and the sealing plate are assembled together, the mounting convex portion is fitted in the mounting concave portion. By merely pushing the mounting convex portion in the mounting concave portion, the sealing plate and the piston can be integrated, which improves productivity in assembly. Further, the sealing plate will not get twisted, deviated or displaced due to friction against inner wall of the pipe cylinder when the sealing plate and the piston are integrally inserted in the pipe cylinder. Deterioration of sealing function due to improper mounting of the sealing plate can be avoided.