To provide an airtight member and its construction method capable of fulfilling the requirements of securing both the airtightness and durability of groundsill and floor joist against water vapor exhausted by means of a simple configuration offering simple work and excellent workability.

Two or more rod-shaped or pipe-shaped elastic packing members 2 are laid in parallel with each other at the end of a sheet 1 with a proper size which has both the moisture permeability and waterproofness. The work method for the airtight member comprises a process of fixing an end of the side where the elastic packing member 2 of the sheet 1 is set on a footing 10 by placing the elastic packing member 2 to the bottom side and arranging an end portion at the opposite side of the sheet 1 to the outdoor side; a process of installing the groundsill 11 on the end portion of the sheet 1 fixed on the footing 10; a process of laying the sheet 1 back to the indoor side in a manner of wrapping the groundsill 11; a process of erecting a column on the groundsill 11 wrapped by the sheet 1; and a process of raising up and fixing the end portion of the sheet 1 at the indoor side.