PURPOSE: To correct the inclination of a truck bed by detecting the inclination and the acceleration of the transverse direction of the bed during traveling, calculating the posture control quantity with which no overturn of the bed is caused when the detected value exceeds the set value, and realizing the drive control of a bed inclining means, and securely prevent the overturn by the lateral force.

CONSTITUTION: The load value is detected by the detected value of a displacement sensor 3 when a truck is in a still condition before traveling. An electromagnetic proportional valve of a hydraulic circuit 23 is set OFF, and a bed 2 is set to the position to be directly supported on a frame 7 by contracting an actuator 6. While the truck is traveling, the respective displacements on the right and the left side which are the inclination of the bed 2, and the acceleration α in the transverse direction are detected by means of the bed sensor 3 and an acceleration sensor 4, and these detected values are received by a controller 5 to make a comparison between the right and the left displacements. When the outer side of the turning circle of the bed 2 is settled due to rapid turning traveling or the like and inclined outward, and when the larger value of the detected displacements is greater than the set value, an electromagnetic proportional valve on the outer side of the turning circle is set ON, and the outer side of the turning circle of the bed 2 is lifted by extending the corresponding actuator 6.