PURPOSE: To obtain a long life rolling diaphragm by providing the intersection of the 1st and 2nd tapered surfaces so that it may be located at the outer part of a diameter direction than a conic surface connecting the inner periphery edge of a flange portion with the outer periphery edge of a plane portion.

CONSTITUTION: When the convolution of a peripheral wall portion 5 rolls as a piston 3 moves, a moving portion 5 free point is set, and given that its diameter, the piston outer diameter, the inner diameter of a cylinder 2 and operation frequency are respectively DA, DB, DC and N, the following relation holds good experimentally. (DC-DB)/DA=alog N+b, but a, b are constants. Therefore, the peripheral wall portion 5 of a diaphragm 1 is constituted with the 1st and 2nd tapered surfaces 5a, 5b, and its intersection is provided so that it may be located at the outer part of a diameter direction than a conical surface connecting the inner periphery edge of a flange portion 4 and the periphery edge of the portion 5. As a result, DA becomes larger, and (DC-DB)/DA becomes smaller, and N increases, and the life of the diaphragm can be extended.