PURPOSE: To lower a cost by placing a soil-sand separating device in a pit with injected stabilizing solution, via a buffer member, in a floating state.

CONSTITUTION: Ground is excavated in a recessed shape, and on its internal surface, a water-proof sheet 14 is laid, and a pit 12 is formed, and in the pit 12, stabilizing solution A is injected. Then, a box-shaped body 16 with a plurality of buffer members 18 set firmly on the lower surface is permitted to float in the pit 12. Besides, when an earth separating device 10 is set in the box- shaped body 16, then the buffer members 18 are butted to the bottom surface of the pit 12 in a state that the soil-sand separating device 10 is placed in the box-shaped body 16.