To couple compactly foldable structural elements by forming a substantially spiral flexible wire having a central axis in the direction where a part stretching across the structural elements couples the structural elements and then securing the structural elements to the stretching part through securing metals.

Collective wires 3 inserted with flexible wires and stretched across structural elements 1 are shaped spirally at a part 3A stretching across the structural elements 1. A line 5 perpendicular to the central axis of rotation 4 at a coupling element 2 is the central axis of the spiral stretching part 3A. When the collective wires 3 are secured by securing metals 10, securing angle and position of the collective wires 3 are adjusted selectively such that the spiral shape is substantially confined between the central faces of adjacent structural elements 1 thus avoiding troubles, e.g. breakage, of the flexible wires.