A sensor and monitoring system that can prevent the accumulation of scraps of a transport object on the sensor are provided. A sensor 4 is located below a return-side portion 2a of a conveyor belt 2, and includes: a detector 40 that detects a detection object 3 embedded in the conveyor belt; and a cover 41 covering at least an upper part of the detector, wherein the cover has a downward surface 411 that extends continuously downward from a top part of the cover while extending outward in the horizontal direction.