generated of course the length of the string 41 to which the spreader 40 is hung can be known by installing the encoder to the hoist for adjusting the height of the spreader 40 fig9 is a top plan view of the spreader where the skew is generated as shown in fig9 a skew angle is the angle formed between a side of the initial spreader 40 indicated in a dotted line 40a and a side of the spreader 40 where a skew is generated indicated in a solid line 40b the skew angle is obtained by the variation of edges of a constant point of the spreader 40 and the distance between the center of the spreader at the point that is to say the variation of the edges of the constant point of the spreader 40 is measured by the position detector and the distance between the measuring point and the center of the spreader 40 is measured thereby enabling to know the skew angle of course in order to know whether only the sway is generated in the spreader 40 or not the variation of the edges of two left and right ends should be detected as shown in fig9 fig1 is a top plan view of the spreader in which a sway and skew are simultaneously generated in this case the average moving distance 42 of the spreader 40 generated by the sway is known also by measuring the variation of two edges with.