PURPOSE: To easily recognize the etching of a diaphragm in a predetermined thickness by using a first groove formed on one side surface of a substrate and a second groove etched simultaneously upon forming of the diaphragm oppositely to the first groove.

CONSTITUTION: After a piezo resistance element 2 is formed on the surface of a substrate 1, silicon oxide films 3, 4 are formed on the front and rear surfaces of the substrate. After windows 5 7 for forming grooves are then formed on the films 3, 4, the substrate 1 is etched by the windows. The etching through the windows 5 is stopped when the depth arrives at the thickness lof a diaphragm 11 to form a first groove 8. The etching of a window 6 for forming a diaphragm and a window 7 is continued until a second groove 10 by the window 7 arrives at the groove 8. When the groove 10 arrives at the groove 8, the substrate 1 is split to finish the etching. At this time, it can recognize that the diaphragm 11 of the thickness l is formed.