To provide a diaphragm pressure sensor which can be manufactured easily at a low cost without trouble and restriction, by simultaneously joining a header to a diaphragm and the diaphragm to an adaptor by projection resistance welding.

The diaphragm pressure sensor includes a header in which an oil storage chamber is formed, a diaphragm which can cover this oil storage chamber of the header, and an adaptor in which a pressure receiving chamber is formed capable of covering this diaphragm. A ring-shaped projection for projection resistance welding is formed on the header in the outer periphery of the diaphragm or on the diaphragm side of the adaptor. Then, this ring-shaped projection for projection resistance welding and the diaphragm, as well as the diaphragm and the adaptor or the header, are simultaneously joined by projection resistance welding to constitute the diaphragm pressure sensor.