To provide a technique of constructing an underground structure in the soft ground for use as a free water underground storage facility, and constructing a building on the underground storage facility as a foundation structure.

There is provided a method of constructing the underground storage facility in the soft ground which is comprised of following steps. In a first excavating step, the soft ground is excavated down to a first depth, to form a first inclined surface having an inclination angle. In a steel sheet pile sequentially arranging step, steel sheet piles are sequentially arranged on an excavated bottom surface along the inclined surface formed in the first excavating step. In the second excavating step, a second inclined surface is formed between the steel sheet piles arranged in the steel sheet pile sequentially arranging step and the inclined surface formed in the first excavating step, and an area enclosed by the steel sheet piles arranged in the steel sheet pile sequentially arranging step is excavated. In the facility constructing step, a storage facility is constructed in the excavated area enclosed by the steel sheet piles. In the back-filling step, back-filling is carried out.