To provide a box capable of being easily assembled and preventing flaps from spontaneouslyopening after the box is assembled.

A body part 2 has an rectangular opening in upper and lower parts thereof, respectively. One of the openings has two first opening edges 8a and 8b which are parallel to each other, and two second opening edges 12a and 12b. First flaps 16a and 16b continue from the first opening edges 8a and 8b, respectively, and are folded along the first opening edges 8a and 8b to close the openings. In the flaps 16a and 16b, arcuate insert openings 18a, 18b, 18c, and 18d are formed inwardly from the outside of both the opening edges 8a and 8b. These are convex to the side of the opening edges 8a and 8b. Second flaps 22a and 22b continue from the opening edges 12a and 12b are formed, respectively, and are folded along the second opening edges to overlap onto the upper side of the flaps 16a and 16b. Ends at both ends of the flaps 22a and 22b are inserted into the arcuate insert openings 18a, 18b, 18c, and 18d to intrude under the flaps 16a and 16b.