In order to output a voltage signal to the ECM which corresponds to the applied pressure, the rail pressure sensor must measure the instantaneous pressure in the rail. The fuel flows to the rail pressure sensor through an opening in the rail, the end of which is sealed off by the sensor diaphragm. Pressurized fuel reaches the sensor's diaphragm through a blind hole. The sensor element (semiconductor device) for converting the pressure to an electric signal is mounted on this diaphrgam. The signal tenerated by the sensor is inputted to an evaluation circuit which amplifies the measuring signal and sends it to the ECM.