PURPOSE: To restrict deformation of a diaphragm, and expand it regularly so as to eliminate inversion in its local bending direction by holding the diaphragm between two containers, and bringing its recessed and projected parts in contact with recessed parts of respective containers when the diaphragm is expanded or contracted.

CONSTITUTION: When a diaphragm 1 is expanded or contracted to gas chamber 13 side or fluid chamber 14 side in association with pressure variation in a fluid chamber 14, the diaphragm 1 is brought in contact with either one of recessed parts 8, 9 which are formed on a main body 6 and a side plate 7. At this time, the contact position between respective recess parts 8, 9 is in a symmetric shape matching the projection part 3 or the recessed part 4 of the diaphragm 1, and is in a such a shape as concentric circle recessed parts30 or projection parts 31 continues to each other alternately. As a result, the deformation of the diaphragm 1 is restricted, and expanded and contracted regularly, and no inversion its local bending there of can be generated. It is thus possible to improve durability of the diaphragm 1 and also extend its life.