To provide a coupling structure which is not easily removed.

A folding box 1 is constituted by a substantial square-shaped bottom surface part 11; and four wall surface parts 12, 13 continuous with four sides of the bottom surface part 11 and raised from the bottom surface part 11. Each of the wall surface parts 12, 13 is connected to each of adjoining wall surface parts 13, 12 at one end. An outward extending connector piece 21 is formed continuously at one side of the end part of the wall surface 12. The connector piece 21 has an insertion part 21b and a locking part 21a having a stopper edge 21A extending in a slant direction from the insertion part 21b. A slit 14 is formed near one side of the end part of the wall surface part 13. As the slit, there are provided an insertion slit 14b where the insertion part 21b is inserted and a stopper slit 14a which is continuous in a slant manner from one end of the insertion slit 14b and the locking part 21a is inserted. The insertion part 21b is inserted into the insertion slit 14b to cause a location near the slit 14 to be curved and the stopper edge 21A is abutted against an edge 14c of the exposed stopper slit 14a.