To cut off ground water from an unnecessary aquifer at a low cost and easily in an existing well.

In an existing well pipe 10, screens 11-13 where ground water flows in are disposed in the positions of first to third aquifers 2, 4, 6. Filters 14-16 are formed to filter ground water. In the case of cutting off ground water from the first to second aquifers 2, 4, first an inner pipe 20 is inserted in the existing well pipe 10. Secondly, a filter material 31 such as gravel or the like is filled between the existing well pipe 10 and the inner pipe 20 up to the position of the third aquifer 6. Subsequently, the upper side of the filter material 3 is filled with plug 32 such as clay or the like. Next, the upper side of the plug 32 is filled with grout 33. The grout 33 permeates into the filters 14, 15 through screens 11, 12 to make the filters 14, 15 impermeable. Thus, it is possible to cut off the flow of ground water from unnecessary aquifers 2, 4 into a well.