PURPOSE: To prevent the fatigue breakdown by concentrically providing an annular recess on a diaphragm and mitigating the internal stress of the diaphragm due to deflection with the annular recess.

CONSTITUTION: When a pressing force is applied to the center section of a diaphragm 31 by a valve stem 32, the predetermined deflection is generated. The bending stress in the thickness direction and the stress in the radial direction are generated against the predetermined deflection quantity in the thickness direction of the diaphragm 31, but an annular groove 39 is formed on the diaphragm 31, thus the actual diameter length when the diaphragm 31 is extended becomes long. Accordingly, various stress values such as the bending stress in the thickness direction and the stress in the radial direction are mitigated, and fatigue breakdown is prevented.