A limit detection system includes: a light emitter emitting a laser and fixed to one of two members; a light receiver receiving the laser and fixed to a same one of the two members to which the light emitter is fixed; at least one light-shielding member fixed to another of the two members, disposed on an optical path of the laser from the light emitter to the light receiver, and formed with an opening through which the laser passes; and a computer acquiring an amount of received light by the light receiver and detecting a positional displacement between first and second members depending on a difference between (i) the amount of received light when the laser having passed through the opening reaches the light receiver and (ii) the amount of received light when the laser at least partially blocked by the light-shielding member reaches the light receiver.