To form a hardly permeable layer efficiently and securely at a bottom between water cut-off walls in an underground drainage construction method.

In executing the water cut-off wall 2 a water pumping pipe 3 for pumping underground water is fitted in one water cut-off wall 2A and an injection pipe 4 for injecting water cut-off chemical liquid is fitted in the other water cut-off wall 2B, and the underground water is pumped with the water pumping pipe 3 and the water cut-off chemical liquid is injected with the injection pipe 4, so that the water cut-off chemical liquid is diffused in one direction by ground water flow caused at the bottom between the water cut-off walls and a film shaped or thin plate shaped hardly permeable layer 5 is formed to cross between bottoms of both water cut-off walls. A soil-cement column wall having H shaped steel 6 as a core material is used as the water cut-off wall and the water pumping pipe 3 and the injection pipe 4 are installed along the H shaped steel 6. The water cut-off wall is used as a diaphragm wall and a screen is arranged at the bottom thereof.