To provide a load box stacking structure capable of additionally loading a part of a load placed on a loading floor on an upper stage.

The loading structure of a motor truck is provided with a vertical moving device 120 provided in a position on a part of a load box floor surface for placing a frame 110 and capable of disposing the frame 110 in the upper part in the lowered position of a mounting platform 129 and freely moving the frame 110 to an upper stage position in the raised position of the mounting platform 129, a frame receiving base 122 for receiving the frame 110 raised to the upper stage position by the vertical moving device 120, and a hydraulic cylinder for fixing the frame 110 placed on the frame receiving base 122. Also, a self-propelled roller conveyor 126 for sliding and moving the frame 110 in the back and forth direction of the load box may be provided on the floor surface.