PURPOSE: To keep joint flaps from being seen from both outside and inside to improve the appearance by a method wherein the joint flaps are inserted between front and rear first side walls and second side walls on the left and right sides and bonded.

CONSTITUTION: Adhesive 9 is applied on the rear faces of left and right first side walls 3, and left and right second side walls 5 are folded and bonded. Joint flaps 7 are overlapped on the underside of front and rear first side walls 4. Adhesive 11 is applied on the rear faces of the front and rear first side walls 4 and front and rear second side walls 6 are folded and bonded. The joint flaps 7 are inserted in between the front and rear first side walls 4 and the front and rear second side walls 6. Adhesive 15 is dotted on the rear faces of the joint flaps 7. The joint flaps 7 is inserted into gaps between the front and rear first side walls 4 and the front and rear second side walls 6 and bonded with the adhesive 15, so that a tapered box 1 of a frustum of a pyramid is formed. Thereby, the joint flaps 7 are not seen from both inside and outside, so that the appearance is improved, resulting in an enhancement of merchandise values. In addition, the joint flaps 7 are kept from being caught by others, so the box 1 is easy to assemble.