PURPOSE: To enable setting of the floor surface of a load compartment of a truck assembled with rear body mobile mechanism downward as low as possible by assembling both cross and main sill, extending in the car width direction and in the length direction, crosswise in two parallels, so as to make their top faces to be flush with each other on the rear body, and supporting a rear body mobile mechanism on the top side of this two-parallel body.

CONSTITUTION: A cross sill 12 and a main sill 11 are assembled on a rear frame 6 crosswise in two parallel so as to make their top faces to be flush with each other, setting them down to a parallel-cross body 1, and a rear mobile mechanism 2 is installed on the parallel-cross body 1 between both rear-body side walls 4 and 4 erected at both sides of the cross sill 12 extending over the full width of the car. This rear- body mobile mechanism 2 is composed of a pair of side frames 26 installed after contacting with an inner wall of the rear-body side wall 4, a closed plate-form aluminum frame 21, whose both sides in the short-hand direction are supported by these side frames 26, and functioning as a rear-body floor surface, and a rubber belt 23 being rounded in a U-shaped form along a lower surface of the aluminum frame 21 after being installed toward the rear end side of a load compartment along the top face. In this rubber belt 23, a partition wall 23 is erected in the part.