PURPOSE: To enhance the durability of an underground structure by forming it from slurry filling a void generated in the ground, and preparing the slurry by mixing bentonite with a solvent containing water-soluble organic solvent.

CONSTITUTION: An underground structure 1 is formed by filling a void formed in the ground G to a certain depth with bentonite slurry 3. In execution of works, the ground G is excavated to form the void 2, to which the subsoil water flows in from the surrounding ground G. Using a pump, etc., the bentonite slurry 3 is put into the water which has flowed into the void 2. The slurry 3 comes in contact with the subsoil water so that a low permeability layer is formed. The bentonite moistens and swells and the slurry 3 attaches tightly to the inside surfaces of the void 2, and thus the underground structure 1 is accomplished in the void 2.