PURPOSE: To provide a light cargo box by making portions to be reinforced of composite members of steel and light members and portions not to be reinforced of only light members.

CONSTITUTION: A floor plate 2 is formed on a composite member including the upper layer 9 made of steel material and stuck to the lower layer 10 made of light member with adhesive, and a rear dump door 6 is formed on a composite member of an inner layer 11 of steel material and an outer layer 12 of light member stuck together with adhesive or the like. Further, side dump doors 5 and fixed posts 3 are formed of composite members of steel material inner layer and light member outer layer similarly to the rear dump door 6. On the other hand, since a front panel 4, main girder 7 and reinforcing girder 8 do not touch cargos and little wear even if they touch the cargos, they do not need so much strength and are all formed of only light members.