PURPOSE: To cause a flexible filler and impermeable cutoff sheets to follow deformation of the ground by disposing the cutoff sheets in a channel excavated in the ground, and filling the channel with the filler covering the peripheries of the cutoff sheets.

CONSTITUTION: Prior to excavation of a channel 12, vertical shafts 15 are drilled at predetermined intervals from above the ground G to the depth of an impermeable layer past a permeable layer, and the channel 12 is excavated between the vertical shafts 15. Cutoff sheets 13 are built into the channel 12. The ends of the cutoff sheets 13 built into the channel 12 are connected together inside the vertical shafts 15. After the cutoff sheets 13 have been built into the channel 12, the channel 12 is filled with a filler 14 in such a way that the filler covers the peripheries of the cutoff sheets 13.