PURPOSE: To eliminate a grouting work and a grout feeder for a cut-off wall, by making a roll of sheet having a pair of fitting pieces and attaching it in a groove buried in the cut-off wall.

CONSTITUTION: After a pair of first grooves 10 where partitions 46 are disposed mutually at a distance are excavated in the ground, sheets 12 having fitting pieces 22 are inserted in the longitudinal direction of the cut-off wall and a filler is charged in the first groove 10. And, then a second groove 60 is formed to connect to the first groove 10. Next, a sheet 62 having a fitting piece 28 which can be joined with the fitting piece of the sheet 12 is inserted in the second groove 60 to connect the sheet 12 and the sheet 62. Thereafter, a filler is charged in the second groove 60 to construct the cut-off wall.