A sensor device includes a base wall, a plurality of sidewalls cooperating with the base wall to define a receiving space that has an open end, an emitter disposed in the receiving space and having an emitter element to emit a light signal, a receiver disposed in the receiving space and having a receiver element to receive the light signal, a cover body disposed in the receiving space between the emitter and the receiver, a ball chamber formed between inner surfaces of the base wall and the cover body, a ball member disposed movably in the ball chamber, an emitter light passage optically communicating with the ball chamber and the emitter, and a receiver light passage optically communicating with the ball chamber and the receiver. The cover body, the emitter, and the receiver cooperatively close the open end of the receiving space.