PURPOSE: To lightly transfer a cargo in a cargo box with small resistance by arranging a chain conveyor on a floor plate of the cargo box in a freight vehicle as well as to use the cargo box as an ordinary cargo box by nullifying the chain conveyor.

CONSTITUTION: A chain conveyor C is arranged on a floor plate 6 of a cargo box 2. In this conveyor C, a part of a chain 13 is composed of a high type chain 13H, and a residual part is composed of a low type chain 13L. When the high type chain 13H moves to the floor plate 6 side, since this becomes higher than a deck surface of the floor plate 6, a cargo is placed on it, and can be transferred. When the low type chain 13L moves to the floor plate 6 side, since this sinks under the deck surface, it does not become a hindrance to cargo loading.