The present invention includes: an independent tank constituting an inner tank to store a storage material therein; at least one sandwich plate modularized and manufactured include a metal plate provided in a pair opposite to each other, the metal plates having a reinforcing material formed therebetween, and a filler filled between the metal plates, the at least one sandwich plate surrounding the outer surface of the independent tank to constitute an outer tank; and an external reinforcing member formed on the outer surface of the sandwich plate.