A connecting structure comprises a tubular connection joint and a tubular pipe-shaped cardboard tube for integral engagement with the tubular connection joint. The tubular connection joint is generally curved-shaped and has an insertion portion and a lock projection disposed along a curved inner surface of the insertion portion. The pipe-shaped cardboard tube has an oblique front end portion configured to be inserted into the tubular connection joint so that the oblique front end portion engages between the lock projection and the curved inner surface of the insertion portion.