PURPOSE: To promote productivity and execution efficiency by providing an auxiliary beam to an upper floor building unit, and connecting it to a ceiling girder of a lower floor building unit in the case the upper floor building unit is projected to mount it on the lower floor building unit.

CONSTITUTION: An upper floor building unit 11A having large size projected outward from a lower floor building unit 11B is mounted on the unit 11B. After that, in an external from surrounded by a floor girder 20 of a floor structural body, the unit 11A has an auxiliary beam 31 in a position corresponding to a ceiling girder 25 of the unit 11B. Then, the auxiliary beam 31 is connected to the ceiling girder 25 of the unit 11B. In addition, in the external frame of the floor structural body, the unit 11A mounts bracings 41A and 41B between the auxiliary beam 31 and floor girder 20 corresponding thereto. According to the constitution, factory productivity can be promoted, at the same time, built-up execution efficiency in a construction site can be promoted, structural strength of the building can be ensured, and flexibility of planning of an upper floor room can be promoted.