PURPOSE: To make the thickness of a membrane thin, eliminating the problems about buoyancy and the strength against water pressure, by allowing a thin-membrane tank to retract in absence of liquid in the thin-membrane tank, when liquid is stored in the tank formed of soft thin membrane in water.
CONSTITUTION: When liquid 20 is introduced through a valve 19 and a fixing pipe 11, a thin-membrane tank 6 swells, and liquid 20 can be stored to the full of the inner volume. At this time, the thin-membrane tank 6 is protected by a tank protecting membrane 5 for the swelling over the maximum permissible volume or the injuries due to external force. When liquid 20 is discharged outside, the thin-membrane tank retracts according to the discharged amount, and according to said retraction, water gets into the protection membrane 5 from a water-through hole 7, and the liquid pressures inside and outside the thin membrane 6 are balanced. A protection box 10 having liquid permeability is formed in the protection membrane 5 so that the thin-membrane tank 6 having retracted does not close the opening part of the pipe 11, and the discharge of the liquid is made easy