PURPOSE: To seal seepage water in a ground by drilling pilot holes in a ground at intervals, forming slits between the pilot holes, and filling grout material so as to provide a water sealing wall in a slit.

CONSTITUTION: A plurality of vertical pilot holes 2 are drilled from a ground surface into a ground 1 at intervals. Also a support member 3 for a wire saw 4 is set on the bottom of the pilot hole 2, and the wire saw 4 suspended from the support member 3 is operated out of the pilot hole 2 so as to form a slit 5. Then a capping 6 to cover the, pilot hole 2 is applied, a filling tube 7 is inserted into one pilot hole 2, and grout material made of cement milk is filled. In addition, when the grout material is overflowed from a return pipe 8, filling of the grout material is stopped so as to provide a water sealing wall. By changing the concentration of the grout material from thin one to thick one in order, the grout material can be filled securely into a small space in a slit 5.