To prevent an airtight film pasted on a floor plate from the end floor girder side from being ruptured by simply supporting the floor plate near the end floor girders.

In this floor plate supporting structure in a building unit 10, a long floor plate supporting metal fitting 40 extending in the longitudinal direction of the end floor girder 11 is attached to each of end floor girders 11 opposed to each other. The airtight film 70 which supports the floor plate 32 and extends from the end floor girder 11 side to the floor plate 32 via the upper section of the floor plate supporting metal fitting 40 is formed on each floor plate supporting metal fitting 40 and a floor beam 20 adjacent to the end floor girders 11 to which the floor plate supporting metal fitting 40 is attached.