PURPOSE: To evenly drain in the whole depth direction, by concentrically fitting a strainer tube and a riser tube in a double-well pipe filled with filter materials in the whole circular space and sucking water in the ground.

CONSTITUTION: In a double-well pipe 10 installed in the ground to suck the underground water and reduce the water content of the ground and increase the strength thereof, water sucked through a strainer tube 6 is collected in the well point 2 through a filter material 4 charged in the circular space and then sacked by the actuation of a drain pump provided with a vacuum pump and discharged out. The filter material 4 is drawn from the strainer tube 6 to the ground level and readjusted. In this way, the ground can be evenly improved in the depth direction.