A flow controller (1), comprising a valve body (4) having flow paths forming an inlet and an outlet of fluid, a diaphragm (6) held fixedly between the valve body and a bonnet (5), and an operating mechanism (7) moving the diaphragm vertically, wherein the valve body flow paths communicate with each other through a projection-free communication path (8), the a diaphragm is formed so that, when lowering, it comes into close contact with the communication path so that it closes the flow path and, when rising, the center part thereof is recessed so that an area near the outer peripheral part is raised, and a curved surface (52), to which the outer upper surface (63) of the raised part is closely fitted, is formed on a bonnet on the inner lower surface of a diaphragm holding part (51).