To endure a heavy load, to simplify a structure, to set accessories and parts, to reduce cost, to carry out extension and contraction work in a short period of time and to reduce an area for the extension and contraction work.

A flexible link part 6 is constituted by connecting a lower end of an upper link part 1 and an upper end of a lower part link part 2 free to bend and extend through an intermediate part connecting body 3 between them, a pair of the flexible link parts 6 is arranged free to bend and extend in the opposite directions, upper ends of a pair of the flexible link parts 6 are connected to each other free to horizontally revolve through an upper part connecting body 7, lower ends of a pair of the flexible link parts 6 are connected to each other free to horizontally revolve through a lower part connecting body 9, an extension working mechanism 22 to flexibly deform an extension mechanism main body 12 is provided by constituting the extension mechanism main body 12 free to extend and contract in the vertical direction by interposing a synchronous mechanism 11 free to bend and extend a pair of the flexible link part 6 synchronously.