PURPOSE: To prevent the danger that the bed of a dump truck moves down due to the shifting of a changeover valve which is caused by unexpected touch on an operating lever or the like, by providing a means capable of being engaged and disengaged between the operating lever and a bracket on the vehicle body.

CONSTITUTION: When an operating lever 8 is lifted in order to elevate the bed of a dump truck, a changeover valve 6 is shifted leftward through a push-pull wire 9 to supply oil from a hydraulic pump 4 to a hydraulic cylinder 3 through a passage 5 to expand the cylinder so that the bed is elevated. To move the bed down, the operating lever 8 is pushed down to shift the changeover valve 6 rightward through the wire 9 to retract the hydraulic cylinder 3 so that the bed is moved down. Since a claw 18 is engaged in a groove 20 to keep the operating lever 8 from turning downward, a button 16 needs to be pressed against a spring 17 to disengage the claw 18 from the groove 20 before the lever 8 is pushed down.