An assembly structure includes a cylindrical part with an end closed on one side. The cylindrical part has three columnar protrusions that protrude from the end to one side and a cylindrical boss portion that is positioned within the cylindrical part such that the three protrusions of another assembly structure are fitted between the boss portion and an inner peripheral surface of the cylindrical part. The inner diameter of the cylindrical part is smaller than the sum of double the outer diameter of the protrusion and the outer diameter of the boss portion. When one protrusion of the other assembly structure is arranged on the outside of the cylindrical part, the two remaining protrusions of the other assembly structure are fitted between the boss portion and the inner peripheral surface of the cylindrical part.