A foldable accommodating box includes a floor having a sidewall accommodating pocket, a pair of end walls pivotally supported on the floor, and a pair of sidewalls capable of being accommodated in the sidewall accommodating pocket in a mutually piled state. Each of the sidewalls includes a wall plate formed into a right-angled quadrilateral shape, a frame plate secured at right angles to an outer surface of an outer periphery of the wall plate, and a plurality of metal bars combined in a lattice shape and secured to the wall plate and the frame plate. The frame plate is provided with a notch for ensuring that when the sidewalls are piled on each other in an embraced manner so that their outer surfaces are opposed to each other, they are piled on each other with the frame plates partially adjoining each other on inner and outer sides. Thus, it is possible to prevent a portion of an accommodated member from protruding, and the sidewall can be constructed to have a sufficient strength, while avoiding an increase in weight. Moreover, it is possible to prevent an increase in thickness of the pair of sidewalls.