Abstract: A pressure sensor has a thin plate diaphragm having a strain detecting section thereon, and a support member for receiving and properly positioning the diaphragm within a stepped bore structure. The pressure sensor with which the strain detecting section is formed on the diaphragm surface is formed by the use of semiconductor manufacture technology by making the diaphragm of the pressure sensor in the form of a thin plate member. The diaphragm and the support member are joined together by diffusion bonding, and heat treatment for this diffusion bonding is also utilized to crystallize the semiconductor strain gauges.