To provide a boom mechanism easily separable and reconnectable for overhauling and unloading.

A lower boom section, an intermediate boom section, and an upper boom section are connected as a derricking motion to each other so as to be raised and lowered by the extension and retraction of the hoist cylinders installed between the connected boom sections. The lower boom section is connected as a derricking motion, at the base end, to a swing column swingable about the vertical axis so as to be raised and lowered by the extension and retraction of the hoist cylinder installed therebetween. The lower boom section is divided into two parts at the generally center position in the longitudinal direction so as to be detachable.