PURPOSE: To prevent the sticking of iron powder or the like on the surface of a magnetizing body by a construction wherein the magnetizing body generating a signal magnetic field is constituted of a magnet and an elastic substance covering the magnet and a gap from the surface of the magnetizing body to the magnet is expanded at the time of non-operation.

CONSTITUTION: In a magnetizing body 9, a magnet 25 (a permanent magnet or an electromagnet) is connected on a fitting base 24, an elastic substance 26 (rubber) is so bonded as to cover the front and lateral sides of the magnet, and a smooth thin plate 27 formed of a non-magnetic metal (Al, about 0.1mm thick) or a non-magnetic substance (fluoric resin) is bonded on the surface thereof. The thin plate 27 is so fitted that the opposite end faces 27a and 27b in the direction X of relative displacement of a magnetic sensor 7 and the magnetizing body 9 are bent onto the base 24 side, and a gap G1 between the surface of the magnet 25 and the front of the thin plate 27 is designed to be about 5mm in the state wherein the sensor 7 is set off the front of the magnetizing body 9. The position of the sensor 7 is adjusted on fitting so that a gap G2 between the surface of the magnet 25 and that of the sensor 7 be about 1mm with the shrinkage of the elastic substance 26 when the sensor 7 is superposed on the front of the magnetizing body 9.