To provide a folding double box, which can be folded flat without forming a folding line to improve the stacking efficiency of the boxes when containing no product, and can retain a high strength to keep the product safe when containing the product.

The double box comprises an outer box formed into a rectangular frame shape by opposing two first side boards, as well as second side boards, each other, and an inner box formed into a rectangular frame shape by opposing third side boards, as well as four side boards, each other, the third and four side boards having sizes allowing the inner box to be fitted into the outer box. Respective bottom boards of the inner /outer boxes are structures such that a bottom strip on the lower end of each opposing side board can be glued and engaged with each other, and folded. On the top of each side board of the outer box, a bendable strip having a prescribed width in the cross-longitudinal direction of the side board and bendable inward is formed, with incisions formed on the boundary of each side board in such a way that each incision extends from the boundary by the distance equal to the thickness of the side board at an angle of 45° or more against the cross-longitudinal direction, and further extends in the cross-longitudinal direction.