a pressure sensor includes a capacitive pressure sensor chip mounted with the intervention of a sealing member on a base adaptor so as to be detachable . the sensor chip being constructed in a manner that a second substrate with a diaphragm electrode is placed between a first substrate with a fixed electrode and a third substrate with a pressure inlet and is bonded to the first and the third substrate so as to overlap the fixed electrode , the diaphragm electrode and the pressure inlet with a prescribed gap between the fixed electrode and the diaphragm electrode . moreover , the third substrate is larger than the second substrate to make place which extends outside the second substrate and plays a role as a sealing surface to be in contact with the sealing member , and the sealing surface is pressed to the base adaptor with the intervention of the sealing member .