A hoist cylinder for rotationally moving a vessel provided between a vehicle body and the vessel, and first and second directional control valves for controlling supply and discharge of a pressurized oil with respect to the hoist cylinder are provided. The first and second directional control valves have a neutral position, a raising position where the vessel is rotationally moved upward, a lowering position where the vessel is rotationally moved downward, and a floating position where free fall of the vessel is allowed by contracting the hoist cylinder by the weight of the vessel. A flow control valve is provided between the first directional control valve and a tank, which controls a flow rate of the pressurized oil discharged from the hoist cylinder into the tank when the first directional control valve is set at the floating position and the vessel freely falls due to its own weight.