A plurality of grooves 25 are formed in an outer peripheral surface of a cylindrical rubber magnet 17. The grooves 25 extend in a direction in which the cylindrical rubber magnet 17 is inserted into a yoke 3. The grooves 25 are open on both ends in the direction of insertion and are also open outwardly in a radial direction of the cylindrical rubber magnet 17. The grooves are formed at predetermined intervals in a peripheral direction of the cylindrical rubber magnet 17. When the cylindrical rubber magnet 17 is inserted into the yoke 3, an adhesive 15 gets into the grooves 25 without being pushed out of a lower end surface 21 of the cylindrical rubber magnet 17. The adhesive located between adjacent grooves also readily gets into the grooves 25. By presence of the adhesive got into the grooves 25, a necessary and sufficient amount of the adhesive can be uniformly interposed between a peripheral wall section 11 of the yoke 3 and the cylindrical rubber magnet 17.