PURPOSE: To shorten working time by bringing a projected member projected out of the end face of one column into contact with a locking member provided in the inside of the end face of the other column, and thereby connecting them to each other.

CONSTITUTION: An upper end face plate 9 is fixed onto the apex of a column 4 in a lower floor building unit 1A, and a projected member 11 having a head section 10 in a truncated cone shape is erected out of the upper end plate 9. On the other hand, a lower end face plate 15 is fixed onto the bottom section of a column 4 in an upper floor building unit 1B, an insertion hole 16 penetrating through the head section 10 of the projected member 11 is formed in the lower end face plate 15, and a locking member 17 is rested thereon in such a way that it can be moved in the radial direction. The upper floor building unit 1B is then rested on the lower floor building unit 1A, the head section 10 of the projected member 12 is inserted into the locking member 17, and concurrently a fastening band 19 is fastened thereon. Therefore, the respective two columns can be easily connected to each other.