PURPOSE: To obtain a great displacement with a small force by providing a flexible backing portion that is arranged close to a diaphragm portion and varies in its shape due to flexure of the diaphragm so as to bear an added load.

CONSTITUTION: A diaphragm assembly 10 has a two-piece structure composed of a diaphragm portion 12 and a flexible backing portion 18. The diaphragm portion 12 works as a fluid barrier for separating two circumstances of a high- pressure side 14 and a low-pressure side 16. The flexible backing portion 18 is disposed close to the diaphragm portion 12 to bear a load generated by a pressure difference between the two circumstances. The diaphragm 12 is made of a suitable non-reaction material, such as 316 stainless steel. A generated stress level is determined by a shape of the flexible backing portion 18 and depends on its rigidity. Since the flexible backing portion 18 is not exposed to a gas flow, it can be made of a metal or non-metal material, e.g. 17-7 stainless steel, which has stronger durability than the diaphragm portion 12. As a consequence, a great displacement can be obtained with a small pressure.