PURPOSE: To suppress variation of span due to tightening of a cover flange without increasing the size by providing a seal ring and the body with inclination.

CONSTITUTION: The seal ring 20a and the body 1a are inclined by such a extent as the elongation of a pressure receiving diaphragm 2(3) is offset in the inner and outer circumferential directions when a cover flange is secured to the body 1a by means of bolts. In other words, the surface where the body 1a comes into contact with the sealing 20a through the pressure receiving diaphragm 2(3) is inclined upward from the inside toward the outer circumference. A force applied through the cover flange and bolt serves as the tightening force. The force spreading outward is offset by the force slipping inward along the inclined surface thus suppressing the shift of fixed point on the diaphragm 2(3). Consequently, the tensile force being applied to the diaphragm 2(3) is reduced and the variation of span is suppressed.