To prevent the collapse of a groove hole, and efficiently construct a cut-off wall in the ground by vertically drilling the groove hole having a prescribed depth in the ground, and then delivering and laying a cut-off sheet while horizontally drilling the groove hole continuously thereto.

A groove hole having a prescribed depth is drilled vertically in the ground in advance by the chain cutter 21 of an excavating device 20. The groove hole is continuously drilled by a plurality of cutters 22 while moving the excavating device 20 in the horizontal direction orthogonal to the groove hole, and at the same time, a cut-off sheet 24 is also delivered from a sheet drum 23 and laid. The edge part delivered first of the cut-off sheet 24 is fixed to a steel bar 27 fixed to the groove hole followed by laying. After the excavating device is removed, a solidifying agent such as cement milk is filled in the groove hole having the cut-off sheet 24 laid thereon, whereby a cut-off wall is completed. The excavation of the groove hole and the laying of the cut-off sheet 24 are simultaneously performed, whereby the collapse of the groove hole is prevented, and the cut-off wall can be efficiently constructed.