PURPOSE: To construct a continuous cut-off wall under the ground whose upper part is a gravel layer and the lower part is a rocky layer by a method in which a pit is formed in the gravel layer by using a casing and a pit is also likewise drilled in the rock layer.

CONSTITUTION: A pit is formed in a gravel layer 1 by using a casing 3 and an excavator 4, and a pit is also drilled in a rockbed 2 by an excavator 6. A water shield material 8 is packed into the pit 7 of the rockbed 2, and send or water shield material 9 is packed into the pit 5 of a gravel layer 1 while pulling out the casing 3 to form a water cut-off body 10 continued from the rockbed 2 to the ground's surface. After the body 10 is serially formed in the planar direction, sheet piles 12 are driven into the body 10. Since the pits 5 and 7 can be exactly excavated and the continuous cut-off wall can be constructed even in the rockbed layer 2, the inflow of groundwater can be exactly prevented.