The invention provides a capacitive pressure sensor with a diaphragm not easily deformed in an initial stage, a pressure detector and an input device. A dielectric film (33) covers the upper part of a fixed electrode substrate (32). A groove (34) is formed on the upper part or the lower part of a compressive stress membrane (38). An upper substrate (35) is stacked on the dielectric film (33). The area located inside the upper substrate (35) and opposed to the groove (34) is provided with a diaphragm (36) capable of elastic deformation. The compressive stress membrane (38) is formed on at least a part of the upper part of the diaphragm (36).