To provide a piston device using a diaphragm membrane resistant to high pressure, free of leakage, and air-tight for a long period to produce a great stroke.

The membrane is supported by a number of through-holes, cavities and irregularities. The suspension area of the membrane is reduced to reduce its pressure receiving force and reduce the thickness of the membrane. A piston is divided into a plurality of telescopic parts which are stored in a telescopic manner to be expanded in sequence with their own weights or which are connected to each other in sequence with springs different in strength to be expanded.