A construction for mounting a pressure detector in a mounting hole of a fixture main body, the fixture main body being mountable in a pipe line and having a fluid passage for admitting a fluid pressure in the pipe line into said mounting hole, the pressure detector comprising a diaphragm base having a diaphragm and a sensor base fastened to the diaphragm base and having a sensor element that is activated with displacement of the diaphragm, the sensor base having a collar overlaying a top surface of the diaphragm base, the diaphragm base and the sensor base being fixed together, a metal gasket of generally rectangular section, said fixture main body defining a first step portion and a second step portion, said gasket being disposed on said second step portion, said diaphragm base having a protruding portion extending toward said second step portion so as to form a seal surface contacting an upper surface of said gasket, and a presser for contacting and pressing on an upper surface portion of the collar on the sensor base so that said seal surface is pressed against said upper surface of said gasket, the upper surface of the collar on said sensor base having a shallow ring groove therein located inwardly of the surface portion contacted by said presser, the diaphragm base having a lower surface with a shallow ring groove therein at a position inward of the protruding portion, the protruding portion of the diaphragm base having inwardly and outwardly facing surface, the inwardly and outwardly facing surfaces each having a shallow groove therein, the shallow grooves in the inwardly and outwardly facing surfaces being opposite to each other, at the position upward of the seal surface protruding downward, whereby strain arising from pressing on the upper surface portion of the collar on the sensor base is absorbed by the shallow grooves.