PURPOSE: To provide a box formed of a body of double-wall structure and a lid part which together lessen the material and yet makes the product as strong structurally as ever.

CONSTITUTION: Relating to both the body and the lid two opposed double side walls 11 are formed to a usual height and the other two opposed side walls which are also double walls are formed substantially to half a usual height and each with cuts in the wall. The double side walls 11 having a usual height are erected by raising opposed side boards 3 and a side board 2 and an inwardly bending board 5 in a manner of tucking in folding boards 4 against the side boards 3. The double side walls with cuts are erected each by folding an inwardly folding side board 3b over an outer side board 3a in a manner of holding a supporting board 7 in between. The body and the lid are formed in sizes suited for fitting them together. The size of paperboard required can be made smaller in width, wet the product does not impair the structural strength by virtue of the double side walls of usual height in use.