To provide air-tight construction for preventing the airflow of the atmosphere flowing through the floor below by making air-tight of a joint space of a ceiling beam on the rear side of a staircase and a floor beam or a wiring space and a construction method for the air-tight construction.

Filler 72 is mounted in a metal floor beam 22 having a U-shaped section in an upstairs building unit, filler 71 is mounted in a metal ceiling beam 13 having a U-shaped section in a downstairs building unit, and a buffer 8 having elasticity is mounted between the ceiling beam 13 and floor beam 22. The upstairs building unit is installed on the downstairs building unit, a floor material in the vicinity of the floor beam mounting the filler 72 therein is notched, a ceiling material in the vicinity of the ceiling beam 13 mounting the filler 71 therein is notched, and stairs 6 are mounted to positions to be notched.