A body defining a concave valve chamber having a valve seat at a bottom thereof, said valve chamber being interconnected to an inflow passage and an outflow passage; FILTER SERIESa metallic main diaphragm provided in the valve chamber opposite the valve seat for maintaining airtightness of the valve chamber and being elastically deformable in a vertical direction to seal and unseal the valve seat and thereby disconnect and connect the inflow and outflow passages; a bonnet fixed to the body for securely holding an outer circumferential edge portion of the main diaphragm against the body; Filterand a stem supported by the bonnet to move up and down therethrough for elastically deforming the main diaphragm; wherein at a top surface of said main diaphragm is arranged a metallic sub-diaphragm having an outer circumferential edge portion which is elastically deformable in the vertical direction, and a center portion which is fixed relative to a center portion of the main diaphragm, so that the sub-diaphragm can contact the top surface of the main diaphragm to support a bend of the main diaphragm when it is elastically deformed.

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