PURPOSE: To reduce underground water pressure, and reduce a spring water quantity reliably in an underground cavity by arranging a grout zone having a necessary inside diameter around a cavity of an underground structure, and arranging drain pipes in a radial manner therein from the underground structure.

CONSTITUTION: A grout zone 12 having an outside diameter of about 1.8D is arranged around an underground cavity 11 such as a tunnel, and drain pipes 13 are arranged in a radial manner in the grout zone 12 by penetrating through lining concrete 14. Next, the drain pipes 13 having a large number of water holes 13b at necessary intervals in a side wall 13a, and are inserted in a drain hole 16 bored in the grout zone 12 through a through-hole 15 from the through- hole 15 formed beforehand in the lining concrete 14. Next, an annular fixing cap 17 formed integrally with an outer flange is fitted to the inserting end of the drain pipes 13, and the fixing cap 17 and a collar 18 arranged fixedly inside of the through-hole 15 are joined together by epoxy resin 19. Underground water is discharged by the drain pipe 13. Thereby, the underground water pressure in the grout zone 12 can be reduced.