PURPOSE: To simplify structure by pivotally mounting the detachable hook to a connecting member projected to a freight box rotatably to the rear section of a chassis and connecting a connecting rod, to which a cylinder is fitted, and an elastic member between the hook and the chassis.

CONSTITUTION: When the freight box 6 is unloaded between the chassis 2 and the ground, a cylinder 14 for the hook is lengthened to backward incline the hook 10 when a changeover valve 27 is changed over to the a direction, and the freight box is unloaded under a condition that the hook 10 and the connecting member 9 of the freight box 6 cannot be connected. That is, a hoist cylinder 5 is shortened, and the freight box 6 is lifted and moved through the revolution of an arm 3, and loaded onto the chassis 2. When the freight box on the chassis 2 is dumped, on the other hand, the changeover valve 27 is changed over to the b direction, the cylinder 14 is shortened to erect the hook 10, the valve is returned to a neutral position, and the cylinder 5 is stretched under a condition that the hook 10 is connected to the connecting member 9. Accordingly, the freight box 6 is inclined through the connection, and dumped.