A boom has a central connection plate attached between a pair of side plates, and a rear portion side of a cover plate on a front boom portion and a front portion side of a cover plate on a rear boom portion are connected thereby. A pivoting support for a working tool cylinder used to move a working tool connected to a foremost portion of the front boom portion is provided on a front side of a top portion on each of the left and right side plates. The central connection plate is formed with a connection plate projection projecting from a front portion attached to the rear portion of the cover plate toward a midway portion so as to approach the pivoting support.