PURPOSE: To make the wing foldable when not using, by coupling the spreader dismountably while constructing the mutual coupling section of bone members such that the angle between each bone member is variable.

CONSTITUTION: When coupling each coupling section 6, 7, 8, it will produce predetermined cubic shape. When not using, a bolt 9 is withdrawn at any one coupling section 8 to decouple the spreader 4 and the coupling tool 82 while loosen the stopper member 73 and screwed section 76 at the coupling section 7. Then the coupling tools 71, 72 are moved along the keel 2 and folded resulting in a compact rod.