PURPOSE: To provide a structure whereby joining operations can be carried out without requiring a large remaining part of surface members and workability can be enhanced.

CONSTITUTION: Partition spaces 10, 11 are formed respectively at the upper end of the pole 2 of a lower unit box 1A and at the lower end of the pole 2 of an upper unit box 1B located above the pole 2 of the box 1A. Each partition space 10, 11 is formed when the inside of the pole 2 made of a rectangular steel pipe is partitioned using end plates 6, 7 and partition plates 8, 9. Each end plate 6, 7 is provided with an anchor through hole 12, 13. An anchor member 5 is provided which is passed through the anchor through holes 12, 13 and enters the partition spaces 10, 11 of the upper and lower poles 2, 2, and each partition space 10, 11 is filled with grout 21. An anchor member 2 has a seat panel 17 at the middle of a deformed steel bar 16 and has a temporary locking claw 18 provided on its lower surface. A joint plate 19 for joining right and left unit boxes together is stacked atop the seat plate 17.