BOOM EXTENDING/CONTRACTING DEVICE

PROBLEM TO BE SOLVED: To provide a sure cylinder support action without any sliding in a cylinder support part by providing a support member telescopically extending/ contracting and fitting a plurality of stages of support members in the outer circumference of a multistage boom extending/contracting cylinder and transmitting a cylinder thrust to the boom as the extending/contracting drive force. SOLUTION: A support S member which supports a 6-stage boom extending/ contracting cylinder C provided in the axial center of a boom B from an outer periphery comprising respective 1st-6th-stage cylinder bodies C1-C6 corresponding to respective boom bodies B1-B6, and transmits the cylinder extending/ contracting thrust to the 2nd-6th-stage respective boom bodies C2-C6 is so constituted that the small-diameter sides of 6-stage square cylindrical support members S1-S6 similar to the boom extending/contracting cylinder C are positioned in the base end sides of the boom bodies B1-B6 and telescopingly extended/ contracted and fitted into the inverse tapered state. The base ends and the tip ends of the respective support members S1-S6 are connected to the base ends of the corresponding boom bodies B1-B6 and the tip of the cylinder bodies C1-C6 respectively.