Joining the two coupling frames columns that make up the linear expansion mechanism increases the rigidity of the rigid state, to realize a smooth delivery and retraction of the operation of the connecting frame column.
Linear expansion mechanism includes a first connection piece of the plurality coupled bendable (53), flexibly linked plurality of second connecting pieces and (54). First linearly rigid when the second coupling piece are joined together, to return to the bent state when separated. First connecting piece has a body portion (530). A pair of bearing portions (531) are provided in front sides of the main body, a single bearing portion (532) is provided in the rear center of the main body portion. For the first coupling frame which is connected to limit the bending of the surface side from linearly aligned position, the bearing portion of the rear of the main body protruding portion protruding rearward (536) is provided, the body portion between the pair of bearing portions of the front receiving portion for receiving the projections (535) are provided.