A diaphragm unit includes a diaphragm and a diaphragm case. The diaphragm has a center point and an outer peripheral portion. The diaphragm case includes a fixing surface and a regulating surface. The fixing surface supports the diaphragm at the outer peripheral portion thereof, thereby defining a fluid chamber in the diaphragm case. The regulating surface forms an inner surface of the fluid chamber, thereby providing a limit of deformation of the diaphragm. The regulating surface has a convex surface region and a concave surface region. The convex surface region is formed continuously with the fixing surface for supporting the diaphragm at a portion thereof adjacent to the outer peripheral portion. The concave surface region is formed continuously with the convex surface region for supporting the diaphragm at a portion thereof adjacent to the center point. Curvatures of the convex and concave surface regions are the same.