1. A motion limiting device comprising: detection means that detects environmental information around a robot; generation means that generates, based on the environmental information detected by the detection means, a three-dimensional environment model that includes an unobservable area and an observable area and indicates a working environment in which the robot operates, the unobservable area being the area where the environmental information cannot be detected by the detection means, and the observable area being the area where the environmental information can be detected by the detection means; and limiting means that limits a motion of the robot when it is determined that the robot has entered the unobservable area based on the three-dimensional environment model generated by the generation means.