PURPOSE: To construct the main part of a box with corrugated paperboard in a foldable manner and enable fixing an inner bag therein as the construction proceeds.

CONSTITUTION: Four side plates 2, 3, 4, 5 to be erected side by side in a sideways row are set up in the shape of a rectangular case with the bottom closed by bottom boards 11, 12, 13, 14 extending from the respective lower edges of the side plates. First and second edge boards 7, 8, 9, 10 extending from the respective upper edges are bent down inward in a manner of bringing together side end edges and triangular pieces 21, 22 extending from the two side end edges of each of folding pieces 17, 18 which extend respectively from the first edge boards 7, 9 are tucked under the second edge boards in a manner of joining the edge boards by clamping. A tongue 29, 30 projecting from each of the folding pieces is inserted into a fastening slit 31, 32 formed in the upper edge part of the side plate 2 or the side plate 4 so as to hem the main part of the box at its opening 23 and at the sane time to fix the edge of the opening of an inner bag to be contained jointly with the insertion of the tongues 29, 30.