To make it possible to easily adjust the inside height of a box to be able to suppress the up-and-down move of an article in the box on transportation.

An angle-forming folding line 8 is formed on respective end flaps 3, and an angle-forming folding line 9, a cutting line 10, and root-forming folding lines 11 are formed on one of side flaps 5 to form a folding strip 12. When the article is short in height, each end flap 3 is folded along the angle- forming folding line 8 and is closed, and the one of side flaps 5 is cut along the cutting line 10, folded along the angle-forming folding line 9 and the root- forming folding lines 11, and closed along with the other side flap 5. As a result, the folding strip 12 is set along the folded parts of the end flaps 3, maintaining the folded state of the end flaps 3 and the side flap 5, thus reducing the inside height of the box.