PURPOSE: To enable secure control of a moving system, by a construction wherein when a force is exerted on a wire rope, a moment of rotation is generated in an arm supporting a tension pulley, and the other end of the arm is fastened to a fixed detent plate, thereby preventing the pulley from being moved.

CONSTITUTION: When a force in the direction of arrow 20 is generated in the tension pulley 16, the arm 12 is given a moment in the direction of arrow 21, and makes contact with both sides of a slot 13a. At this time, the other end of the arm 12 is fastened to a saw tooth part of the detent plate 13, whereby the arm 12 is prevented from being moved in the direction in which the pulley 16 is pulled, and the tension of a spring 14 can not act. With the force in the direction 20 moved, the tension of the spring 14 acts in the direction of arrow 22, thereby exerting a tension in the direction of arrow 24 on the wire rope 8, and simultaneously, the arm 12 is moved in the direction of arrow 23. This condition is generated when the wire rope 8 is slightly elongated for a long period of time.