PURPOSE: To form an accurate detector wherein initial tension is simply imparted to the diaphragm by providing slits at the opposite sides of the diaphragm of metallic rings to which the diaphragm is welded, and by coupling plate materials to the metallic rings.

CONSTITUTION: The displacement detecting mechanism 118 comprising the diaphragm 111 and fixed electrodes 121 and 122 is provided in a hollow part of a metal body 111. The diaphragm 119 is welded to a pair of rings 125 and 126. The slits 411 and 412 having a depth D are provided at the opposite sides of (a) the diaphragm 119 in the metallic rings 125 and 126. The plate material 123 and 124 having through holes at the centers are coupled into the rings 125 and 126, and the initial tension is imparted to the diaphragm 19. The plate materials 123 and 124 are covered by blocking plates 112 and 113 having through holes at the center. Separating films 116 and 117 are attached to the outside thereof, and non-compressible liquid is injected. The diaphragm 119 is displaced due to the difference in pressure applied on both sides, and the capacity between the fixed electrodes varies. In this constitution, the initial tension is imparted by the simple machining, and the accurate detector can be manufactured.