To provide a permeability confirmation method of the lower ground of a cylindrical vertical type underground tank capable of easily and properly ensuring imperviousness of an objective ground in accordance with a result of a permeability test of the objective ground easily carried out and an impervious method.

A permeability confirmation method of the lower ground of a cylindrical vertical type underground tank and an impervious method include the following processes. (1) An underground continuous wall construction process for constructing an underground continuous wall R with a closed section having through-holes (casing pipes 21 and 22) in the direction of the depth at a specific interval in the objective ground. (2) Among a plurality of through-holes, the front ends of a plurality of optionally selected through-holes are drilled to a specific depth to make a test hole 31, and the test hole 31 is used to execute a permeability test of a plurality of holes. As a result, when a permeable area (water path M) is discovered, a hardener injection process for injecting the hardener into the permeable area by using the through-holes or the test hole.