A rubber magnet (3) comprises: a plate-like tilted section (21) extending in the widthwise direction of a conveyor belt, tilted relative to the front surface (2a) of the conveyor belt (2) in a cross-section of the conveyor belt in the longitudinal direction thereof, and rectilinearly extending from an outer end (3b) which is located on the side close to the front surface (2a) to an inner end (3a) which is located on the side far from the outer end (3b); and a plate-like parallel section (22) continuing to the inner end (3a) of the tilted section (21) and disposed parallel to the front surface (2a). The tilted section (21) and the parallel section (22) are both magnetized in the thickness direction.