PURPOSE: To disperse stress concentration of welds on main beams by extending reinforcing edges on at least both lower edge corners of inner ends of cross beams fixed widthwise of a cargo box to the main beams fixed to the back of floor plate of the cargo box and fixing the reinforcing edges to the main beams.

CONSTITUTION: To the back of floor plate 2 of a cargo box 1 on a damp car are fixedly secured a pair of main beams 5 centrally and longitudinally along the total length of the cargo box 1. Also, a plurality of cross beams 6 are fixed in parallel to the back of the floor plate widthwise of the cargo box 1 to cross the main beams 5 perpendicularly. While these cross beams 6 have a U-shaped section formed by the front and rear vertical plates 7 and bottom plate 8, reinforcing edges 9 are extended at right angle to both lower edge corners of inner ends of cross beams 6 fixed to the sides of the main beams. The cross beams 6 are fused to the sides of the main beams 5 on the reinforcing edges 9, vertical plates 7 and outer sides of the bottom plates 8 to thereby prevent both lower edge corners from stress concentration.