PURPOSE: To prevent the dropping-off of a gate to heighten safety, by letting the hook of the gate and the auxiliary hinge pin of a body frame go off freely at the time of downward opening and by engaging the hook and the auxiliary hinge pin at the time of upward opening.

CONSTITUTION: Since a lockup lever 9 is energized in a releasing direction (counterclockwise) by a spring, the lever 9 is automatically swung in the releasing direction as a body 1 is moved for dumping. As a result, a hinge pin 6 is unlocked. A gate 4 is opened downward because a hook 15 provided on the gate 4 goes off freely from the auxiliary hinge pin 13 of a body frame 3. To open the gate 4 upward, it is horizontally swung centering the pins 6, 13. At that time, the pin 6 is kept locked up and the hook 15 is engaged upward with the pin 13. As a result, the dropping-off of the gate 4 is prevented even if the pin 6 is unlocked, because the forward and backward motion of the gate 4 is restricted by the pin 13 and the hook 15 and the upward and downward motion of the gate 4 is restricted by the pin 6 and a member 18.