side spreader 46 is connected to the gantry 41 a hoisting rope 47 extending from the hoisting winch 7 mounted to the gantry 41 is spread on both spreaders 45 and 46 when the mast 40 is raised or lowered by the winch 7 the boom 4 connected to the mast 40 through the guy line 44 is raised or lowered thus in the boom hoisting device of this second embodiment since the hoisting winch 7 is installed to the gantry 41 the hoisting winch 7 can be removed together with removal of the mast side spreader 45 from the mast 40 and removal of the gantry 41 from the upper rotating body 2 in this way the gantry 41 can be removed easily from the body frame without the need of pulling out the hoisting rope 47 from both spreaders 45 and 46 in this gantry removing work as noted earlier it is preferable that the mast side spreader 45 is drawn close to the gantry side spreader 46 by the hoisting winch 7 to make the mast side spreader 45 difficult to shake during transport the mast 40 may be left removed or mounted in accordance with transport weight as to the form of the body frame when removing the gantry 41 in this second embodiment there are included an integrally combined state of both lower travel body 1 and upper rotating body 2 and a separated state thereof although in the above second embodiment the raising lowering winch 7 is installed on the gantry compressing member 42 side the winch 7 may be installed on the gantry pulling member 43 side to.