End plates are forcibly fixed on the longitudinal end surfaces of pillars such as foundation piles buried in the ground. The upper and lower end plates are butted, and are vertically held by a circumferential recessed groove on the inner surface of an inside ring which is divided in a plurality of parts in the circumferential direction. The inside ring is provided with a conical taper around the outside diameter surface. The conical taper is fastened by an outside ring externally fitted around the outside diameter surface of the inside ring, to join the pillars to each other. This joint structure is simple, and makes it possible to certainly join the pillars at the job site, without any welding. Moreover, there can be provided a tough joint structure against bending.