To connect a building unit, to which a screw bar having length, in which one standarized material is installed, is mounted, and a roof unit, in which the two standardized materials are superposed as connecting materials, easily by screwing a cylindrical nut to the screw bar and screwing a bolt to the cylindrical nut from the upper section of the connecting material.

A through-hole 59 is formed to an upper frame 51 consisting of a standardized material in a wall panel 5, a screw section 71 on the lower side of a screw bar 7 is inserted into the through-hole 59, and a washer 75 is inserted and a nut 78 is screwed. A roof unit 3 is placed on the building unit 2 while the screw bar 7 of the upper frame 51 in the building unit 2 in a second floor is inserted into the through-hole 69 of a connecting material 62 in the roof unit 3. When one standardized material is set up in the length of the screw bar 7 and the screw bar 7 is put into the through-hole 69 of the connecting material 62 composed of the two standardized materials, the screw bar 7 is not projected to an upper section. A cylindrical nut 8 is screwed to the screw bar 7 in the through-hole 69, and a bolt 85 is screwed to the cylindrical nut 8 from the upper section of the connecting material 62, thus simply coupling the connecting material 62 with the upper frame 51.