Abstract: A light-weight, robust, foldable box which can be folded flat, comprises a bottom panel, swing side panels and folding side panels of corrugated plastics board. The bottom panel has vertical panels on both ends and a retaining portion in the vertical panels. The swing side panels are produced of corrugated plastics board used in such a direction that a core member will be vertically positioned, and are provided with a reinforcing band at the lower end and an engaging portion formed in the reinforcing band to thereby engage with the retaining portion. In the upper end of the swing side panel is formed an inside-folding line which allows the swing of the swing side panel.