PURPOSE: To make the form of an inner piston selectable at will, by installing the inner piston interposingly between a piston crown and a piston rod.

CONSTITUTION: An inner piston 3 is shaped in such a form that a part 3a situated at the inside of a piston crown 1 has removed the central part of an ellipsoidal section, and provided with a speel part 3b in its outer circumferential part. And, it is tightly attached to the piston crown with a bolt D1 and also tightly attached to a piston rod with a bolt D2. With this constitution, the form of the inner piston is made selectable at will whereby a thermal stress in the piston and stress due to gas pressure are reducible.