A diaphragm device comprising: a supporting member; a flexible membrane that is supported by the supporting member; and a housing that accommodates the flexible membrane and the supporting member, and wherein the flexible membrane air-tightly separates a first chamber and a second chamber connected to the outside through a through-hole formed in a membrane supporting portion of the supporting member, inside the housing, wherein an avoidance recessed portion to avoid interference with an opening edge of the through-hole is formed at a position corresponding to the through-hole in the flexible membrane, wherein the avoidance recessed portion is provided at a thick portion thicker than the other portion of the flexible membrane, and the thick portion protrudes toward the first chamber.