TELESCOPIC BOOM

PURPOSE: To enhance the safety against buckling of a multi-stage unit by providing a reaction part which is radially movable, which is oriented toward the outer jacket surface of a discharge stage of a piston-cylinder unit and which can abut against the piston-cylinder unit, in a base zone of each of at least few cylinders. CONSTITUTION: A reaction part 7 composed of a ring 20 fixed in the base zone of a second row cylinder 2 is provided so as to surround a piston-cylinder unit 13 with a small gap therebetween. A reaction part 8 corresponding to the stationary reaction part 7 is provided in the base zone of a last cylinder 5. The base zones of cylinders 3, 4 are provided therein with pawls serving as reaction parts 18, 19 which are radially movable and which can abut against the unit 13 in a direction toward an outer jacket of a dischargeable stage of the unit 13, and being movable along the inner peripheral zone, at least three arcuate intervals along the peripheral zone. With this arrangement, it is possible to provide a high bucking strength to the unit 13.