PURPOSE: To prevent the welding inferiority of center diaphragms by moving a seal liquid through the communication hole of the second center diaphragm having a smaller diameter fixed to a first center diaphragm.

CONSTITUTION: When pressure is applied on the high pressure side, the center part of a first center diaphragm 21 is prevented from deformation by the second center diaphragm 24 attached to the first center diaphragm 21 and deformation is mainly generated in the outer peripheral part of the diaphragm 21 to suppress the quantity of the deformation of the diaphragm 21 low. When pressure is applied on a low pressure side, a seal liquid 102 is moved through the communication hole 26 of the diaphragm 24 and the overlapped part of the diaphragms 21, 24 is deformed to obtain the large quantity of displacement. Therefore, the volumetric change quantity based on the deformation of the diaphragms 21, 24 due to the pressures on the high and low pressure sides is made high on the high pressure side and low on the low pressure side in order to obtain definite displacement. As a result, since only one thin diaphragm 21 is held between housings to be welded and fixed, welding and fixing become easy and welding inferiority can be prevented.