In a diaphragm valve comprising a tubular valve seat, a primary side passage located outside the valve seat, a secondary side passage located inside the valve seat, and pressing member which presses the diaphragm to a seating surface to change a valve opening, a supporting member which contacts with the diaphragm in a valve opening range that is at least one part of an entire opening range from a fully opened state to a fully closed state to obstruct deformation of the diaphragm to the secondary side passage side is disposed in a region between the seating surface and a center of the seating surface. Thereby, even when a pressure difference between both sides of the diaphragm is large, a reduction of a gap at the seating surface can be prevented, and gas can be flowed at a large flow rate.