When the diaphragm 15 is of composed a material such as Al2O3, and the radius (r) of the diaphragm 15 is 90 ??m and the film thickness (H) is 3 ??m, the displacement (W) is 0.026, 0.064, 0.129 ??m respectively, when a pressure of 0.2, 0.5, 1.0 bar is applied on the diaphragm 15.