PURPOSE: To prevent a sensor from curving at a small radius of curvature, by providing a protecting member along a sensor, sticking the protecting member to a pipe, and embedding the pipe, the protecting member and the sensor by taping.

CONSTITUTION: A protecting member 12 is provided along a sensor S, and in the position where the sensor does not curve, a pipe P, the sensor S and a protecting member 12 are stuck by a sticking means and are embedded by taping. Then, the radius of curvature of the curved part is determined by the member 12 and the curving of the sensor at the radius of curvature smaller than the prescribed value is obviated. Thus the generation of a deviation between a central conductor 2 and an external conductor 4, hence the change in the electrostatic capacity and impedance of the sensor S and the occurrence of a change with temp. or aging, are prevented.