A mechanical quantity measuring device (semiconductor strain sensor) has a semiconductor chip including a plurality of piezoresistive elements formed on a front surface of a semiconductor substrate, a lead wire unit electrically connected to a plurality of electrodes of the semiconductor chip, and a plate member joined to a rear surface of the semiconductor chip. Further, the plate member includes a first region facing the rear surface of the semiconductor chip and a second region provided adjacent to the first region, and a thickness of the plate member in the first region is made larger than a thickness in the second region.