PURPOSE: To safely construct an underground tank in a short period at a low cost when no impervious layer exists.

CONSTITUTION: An underwater continuous wall 5 is constructed, a base 7 having hollow sections 9 and an opening section 11 is constructed in the underwater continuous wall 5, the sediment under the base 7 is excavated in the underwater while the base is floated, and excavated sediment is discharged to the outside via the opening section 11. When the sediment is excavated to the prescribed depth, concrete is placed on the opening section 11, side walls 19 are constructed, water is drained, a skeleton constituted of the base 7 and the side walls 19 is bottomed, and soil is buried around the skeleton.