A rubber magnet (3) includes a plate-like diagonal portion (21) and a plate-like parallel portion (22) which are both magnetized in a thickness direction, the diagonal portion (21) extending in a conveyor belt width direction while linearly extending, in section in a conveyor belt length direction, from an outer side end (3b), which is closer to the front-side surface (2a), to an inner side end (3a), which is away from the front-side surface (2a), while being at an angle to the front-side surface (2a), the parallel portion (22) being formed continuously from the inner side end (3a) of the diagonal portion, so as to be arranged parallel to the front-side surface (2a).