A construction for mounting a pressure detector prevents the detector diaphragm from being strained by stress applied to the pressure detector as the detector is mounted in a fixture main body provided in a pipe line or the like, thereby keeping the output characteristics and temperature characteristics of the detector from greatly differing before and after the mounting. The pressure detector is constructed by combining and fastening together a diaphragm base having a diaphragm and a sensor base having a sensor element therein that is activated by displacement of the diaphragm base. The pressure detector, with a gasket placed thereunder, is disposed in a mounting hole of a fixture main body that is mounted in a pipe line. The pressure detector is airtightly pressed and fastened by a presser member inserted from above in the mounting hole. The presser member is brought in contact with a block upper surface of the diaphragm base, and the gasket is also brought in contact with the block lower surface of the diaphragm base. A shallow groove is defined in the form of a ring on the lower surface of the block at a place inward of the portion contacting the metal gasket so that the shallow groove absorbs strain caused by the presser member.