PURPOSE: To prevent a measuring diaphragm and a fixed electrode disposed oppositely thereto from being seized in the center thereof upon reception of an undue differential pressure.

CONSTITUTION: Cylindrical recesses 9a are distributed and formed in the center on one side of a diaphragm 9. Cylindrical recesses are also distributed and formed in the center on the other side of the diaphragm 9. Similarly, cylindrical recesses may be made in the surface of a fixed electrode facing the diaphragm 9 or the surface of the fixed electrode may be flat. The cylindrical recesses are made by etching or mechanical machining. Upon reception of an undue pressure, the contact area between the diaphragm 9 and the central part of the fixed electrode facing the diaphragm is decreased because of the plurality of recesses 9a made therein. Consequently, the mutual tightness is reduced, which results in preventing the seizure.