To float an underground structure upward by providing underground walls to surround a fixed region below the underground structure, and displacing the underground walls inward via the water pressure of the outside ground when the ground is liquefied.

When the liquefaction of the slant ground occurs by an earthquake, cutoff walls 5 and partition walls 7 integrally resist the flow pressure of the ground. An underground multi-purpose duct 1 is hollow, the water pressure on the inside of the cutoff walls 5 is smaller than that on the outside, the cutoff walls 5 are pressed and displaced in the direction shown by an arrow (q) by the pressure difference. The space S2 surrounded by the cutoff walls 5 and partition walls 7 has no creephole of underground water, the water pressure is increased, and the increased water pressure acts to the base plate 1a of the underground multi-purpose duct 1 as uplift pressure, therefore the underground multi-purpose duct 1 applied with no pile support is floated upward. The upper end sections 5b of the cutoff walls 5 function as guides when the underground multi-purpose duct 1 is floated.