PURPOSE: To reduce the temperature strain applied to a liquid partitioning diaphragm so as to suppress the drift and temperature hysteresis and reduce the size of the diaphragm by making a closed curve formed by the weld part between a back plate section and pressure receiving section smaller than that of the weld part on the periphery of the diaphragm.

CONSTITUTION: When a temperature change occurs in the ambient temperature, etc., a strain is generated in the weld 24 between a back plate section 22 and pressure receiving section 1B due to the difference in coefficient of thermal expansion between the sections 22 and 1B. The strain is applied to the weld 23 on the periphery of a liquid partitioning diaphragm 21. Therefore, the more the temperature strain applied to the diaphragm 21 becomes smaller, the more the closed curve of the weld 24 between the sections 22 and 1B becomes smaller than that of the weld 23 on the periphery of the diaphragm 21. When the temperature strain becomes smaller, the diaphragm 21 can be used in a wide range as far as the temperature does not exceed the yield point of the diaphragm 21 and the generated drift and hysteresis are reduced. Therefore, the size of the diaphragm 21 can be reduced.