PURPOSE: To sufficient watertightness securable as well as to facilitate a job for waterproof execution operations even in the case where there is little in the build-up value of a waterproof sheet from the top of an appurtenance.

CONSTITUTION: A waterproof sheet 30 is installed in an outer passage unit 1 at a factory, and one end part 13A of a lower frame bracket 13 or a build-up part is installed in a lower beam 7 of a building unit 4 constituting a unit system building 2. In succession, another one end part 17A of a connecting waterproof sheet 17 is attached tight to this one end part 13A, while the other end 17B is hangingly extended and installed therein, and then both these waterproof sheets 30 and 17 are connected to each other by means of welding and bonding or the like at the site.