To suppress and lessen the protruded quantity from a back plate, and moreover easily separate the wear indicator, by holding an inner sleeve through an outer sleeve, and moreover engaging an outer peripheral surface with a claw piece to prevent the extraction from a holding tube.

Inner and outer sleeves 33 and 35 hold the base end part of a holder 13a and the sleeve 33 respectively, and the sleeve 35 is nippedly held from an axial directional side between a second stage part 32 and respective locking parts 34 and 34, to prevent the slipped movement of the sleeve 35 in the axial direction to a holder 13a. Moreover the outer peripheral surface of the sleeve 35 and claw pieces 31 and 31 are engaged to prevent the extraction of the sleeve 35 from a holding tube 27. By thus supportedly fixing the holding 13a to a back plate 5, the protruded quantity of a member, for supportedly fixing a wear indicator 12a, from the inner peripheral surface of the plate 5, can be suppressed less.