PURPOSE: To improve airtightness and decrease heat loss for a building unit by a method wherein long sealing members for sealing gaps between ceiling boards and ceiling beams and gaps between floor boards and floor beams are provided between the ceiling boards and the ceiling beams on the ceiling and between the floor boards and the floor beams on the floor of a building unit along the longitudinal directions of the ceiling beams and the floor beams.

CONSTITUTION: Studs 4 are inserted into notches 7 provided to sealing members 5 and 6. The sealing members 5 and 6 are fixed with nails to floor joists 1 with a face 10 of the sealing member 6 contacting with a gable-side floor beam fitted to the gable-side floor beam 3, a face 11 of the sealing member 6 contacting with the floor joists fitted to the sides of the floor joists 1 and a face 9 of the sealing member 5 contacting with the floor joists fitted to the surface of the floor joists 1. Then, a floor board 2 is fixed with its side fitted to a face 8 of the sealing member 5 contacting with the floor board. Therefore, a gap between the floor board 2 and the gable-side floor beams 3 is sealed up, and inflow and outflow of air to and from the floor surface in the direction marked with R can be prevented and thereby airtightness and heat insulation for a building can be improved.