A condenser sensor (10) comprises an electrically conductive case (20) having an opening portion (22a) formed therein and an opposing portion (22b) opposing to and spaced apart from the opening portion (22a); a fixed electrode (30) received in the electrically conductive case (20) through the opening portion (22a); an electrically conductive diaphragm (51) accommodated in the electrically conductive case (20), the electrically conductive diaphragm (51) spaced apart from the fixed electrode (30) and opposing to the opening portion (22a); an electrically conductive diaphragm supporting member (52) disposed in the electrically conductive case (20) to support the diaphragm (51); a circuit packaging board (60) disposed in the electrically conductive case (20) to be held in electrical contact with the fixed electrode (30) and the diaphragm (51) respectively through the electrically conductive case (20) and the diaphragm supporting member (52); and deformation protecting member (32) for protecting the opposing portion (22b) from being deformed, in which the deformation protecting member (32) intervenes between the electrically conductive case (20) and the diaphragm (51), the deformation protecting member (32) is disposed inwardly of the circumference (51b) of an oscillatable portion (51a) of the diaphragm (51).