PURPOSE: To enable exerting of a working force both in an expanding and a contraction direction, by a method wherein an extrusion wire and retraction wire are worked through the working of a hydraulic cylinder situated in a top boom, tension is always exerted on the two wires to expand and contract a lower boom.

CONSTITUTION: A hydraulic cylinder 39 worked in the direction of length is situated in a small top boom 33, and an extrusion wire 47 located between the inside of the top boom 33 and the tip of a middle boom 32 is reversed by means of a rod 40 of a hydraulic cylinder 39. A retraction wire 45 located between the tip of the top boom 33 and the rear end of the middle boom 32 is reversed by means of the rod 40 of the hydraulic cylinder 39. Tuning wires 35 and 37 are located between the top boom 33 and a base boom 31. This constitution enables generation of respective actuating forces in expansion and contraction in the direction of the length of a boom 10 and reliable expansion and contraction of the boom 10 both in a vertical and a horizontal direction.