PURPOSE: To provide a bottom structure for a rectangular foldable box made of rigid and elastically deformable plastic sheets.

CONSTITUTION: A box comprises first folding pieces 5, 6 and second folding pieces 7, 8 perpendicularly facing the pieces 5, 6. The second folding pieces 7, 8 comprise stepped rims 7c, 8c including wide parts 7a, 8a and narrow parts 7b, 8b, while locking recesses 7e, 8e having a small radius of curvature are provided at corners 7d, 8d formed at middle parts of the rims 7c, 8c. While the second folding pieces 7, 8 are folded perpendicularly inward the box, one wide part 7a and one narrow part 7b overlap the other narrow part 8b and the wide part 8a, respectively, and also the respective locking recesses 7e, 8e are locked to each other. Thus it is very easy to assemble and fold the box, while resistive force against bottom disassembly due to a load of contents is high.