A sensor includes a permanent magnet mounted on a shaft, a magnetic yoke, a magnetism collecting ring, a circuit board, and a sensor housing through which the shaft is inserted. The sensor housing has a box-shaped accommodating chamber accommodating at least the circuit board via an opening that opens in a direction crossing an axial direction of the shaft. A reinforcing member connecting two wall surfaces of the accommodating chamber that face each other in the axial direction of the shaft is provided in the accommodating chamber.