PROBLEM TO BE SOLVED: To provide a method for making it possible to effectively use a sandbag as material of construction which, conventionally, has been disposed of as industrial waste after use and has not been effectively used because the sandbag is distorted with time in the case of being used in the long period of time and, therefore, is not suitable for permanent use in the long period of time.SOLUTION: A method for using a sandbag includes: use of a sandbag obtained by charging a bag having water permeability with inner-filling material having hydraulicity, by pouring water into the sandbag after its installation to harden the inner-filling material； and reuse of the hardened inner-filling material from the sandbag retrieved after use, for another sandbag or reuse of the hardened inner-filling material from the sandbag retrieved after use, as an artificial algal reef installed under the sea.
