To carry out extension/storing operation smoothly and to improve structural strength without forming a gap on an outer circumferential face in the extended state.

The connection structure according to the present invention is provided with a structural unit 1 in which 5 pieces of arm units 2 having an inner arm member 3 and an outer arm member 4 connected in capable of extension through an elbow joint mechanism 5, a shoulder joint mechanism 6 provided at the end of the inner arm member 3, and a wrist joint mechanism 7 provided at the end of the outer arm member 4 are connected with each of the shoulder joint mechanisms 6 at the center radially so as to constitute a frame of a pentagular cone in the extended state so that a gap is not formed on the outer circumferential face in the extended state, extension/storing operation can be made smoothly and strength of the structure can be improved.