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, wherein the limiting means limits at least one of a motion speed, a motion acceleration, and an operational force of the robot, the robot comprises operation means for operating an operation target object and moving means for moving the robot, and upon determining that the robot has entered the unobservable area based on the three-dimensional environment model generated by the generation means, the limiting means decreases at least one of (i) a motion speed limit value that determines an upper limit of the motion speed of the moving means and the operation means, (ii) a motion acceleration limit value that determines an upper limit of the motion acceleration of the moving means and the operation means, and (iii) an operational force limit value that determines an upper of the operational force of the moving means and the operation means.