To prevent the difference of height levels generated between the mutually opposed beams of two building units adjacent in the horizontal direction.
In mutually adjacent first-floor building units 3, an upper-side holding piece 11 is stretched between the top faces 8 of mutually opposed lower beams 7A, 7B, a lower-side holding piece 12 is stretched between undersides 9, a bolt 13 is fixed onto the lower-side holding piece 12, and a block 17 is press-fitted to the root section of the bolt 13. The upper-side holding piece 11 is inserted to the bolt 13 while a nut 14 is screwed. When the nut 14 is clamped gradually, the lower beams 7A, 7B having different height levels are pushed in the direction that the lower beams 7A, 7B reach mutually the same height level.