the area of the electrode plate is 5 0 mm u00d7 5 0 mm and the distance between the electrode plates if 5 um in the pressure sensor of this invention the diaphragm 1 is formed in a deflected or convex configuration whereby it partially contacts plate 2 when no pressure is applied alternatively diaphragm 1 is formed whereby it is sufficiently deflected to contact plate 2 when a reference pressure is applied to the diaphragm during use this deflected configuration is obtained by machining or by the following method after the diaphragm is formed by an si anisotropic etching process i a silicon oxide film is formed by a cvd method on the diaphragms inner surface i e the diaphragm 39 s surface which faces the cavity or ii a silicon nitride film is formed on the outer surface of the diaphragm during formation of the oxide film or nitride film the temperature is kept high but afterwards while the film is cooled to a normal temperature the diaphragm deflects toward the cavity by the internal stress caused by differences between the coefficients of thermal expansion of the silicon film and the silicon oxide film or silicon nitride film in the event a capacitive pressure sensor is desired for measuring absolute pressure of more.