To achieve a wear indicator 7a capable of stabilizing the frequency of a vibration sound, and also effectively achieving prevention of breakage and prevention of damage to a side surface of a rotor 1.

A whole generating line shape in a width direction of a front end edge 15a of a detection part 14a is formed into a projecting arcuate shape of a single circular arc having a constant radius of curvature R_15a, and also a center part m in the width direction of the front end edge 15a is projected the most toward a side of a tip end. Then, the center part m in the width direction of the front end edge 15a and a part in the vicinity thereof are made to slidably contact with a side surface of a rotor 1 by point contact.