PURPOSE: To improve detecting accuracy, by detecting the horizontal displacement of the suspending tool without contact by a laser light receiving device which is provided in the suspending tool.

CONSTITUTION: The light receiving device, wherein light receiving elements are arranged on a two dimensional plane, is provided at the positions facing the laser light on the suspending tool. The output of the light receiving device is inputted to a microcomputer 18 through an adder 15, multiplexers 16A and 16B, and A/D converter 17A and 17B. The microcomputer 18 receives the output of the A/D converter 17A and 17B, computes the center of the laser light with respect to rows and columns, and computes the swinging angles 20A and 20B of the suspending tools.