PURPOSE: To form a side gate board in a rotatable shape even at the outside and the inside by a mechanism wherein the side gate board is mounted at the outside of an upper hinge and a lower hinge is fastened at the outside of a rear body.

CONSTITUTION: When a side gate board 1 is pushed outward by force surpassing the push force of a spring 12, the side gate board is turned outward and tilted using a lower hinge pin 8 as a shaft because the side gate board 1 cannot be rotated in the counterclockwise direction to an intermediate hinge 6. When the side gate board is intended to be raised up, since the intermediate hinge 6 intends to turn in the clockwise direction at all times by the push force of the spring 12, the side gate board 1 and the intermediate hinge 6 are unified, and rotated using the lower hinge pin 8 as the shaft, and the side gate board is raised up. Even when the side gate board 1 is pushed inward, since the intermediate hinge 6 remains as it is contacted with a side floor frame 10 by the push force of the spring 12, the side gate board 1 is turned employing an upper hinge pin 4 as a shaft, contacted with a floor plate 14 and formed in a horizontal shape.