To provide a metal diaphragm valve by which a raw material gas can be stably fed with a gap held constant between the metal diaphragm valve and a valve seat in a high-temperature environment.

The metal diaphragm 15 which performs opening and closing operations of a valve by contacting with a valve seat 14 by a pressing force of a valve bar 16 to return flexibly to an original form by releasing the pressing force of the valve bar 16 is made Hv 500 or higher in Vickers hardness by applying aging heat hardening treatment. As a result, even in a high-temperature environment, the metal diaphragm 15 can be reduced in the deterioration of a reactive force (elastic force) and in a thermal expansion amount and thereby a gap between the metal diaphragm 15 and the valve seat 14 is held constant, so that the raw material gas is stably fed.