To implement a multipurpose water tank which is lightweight, handy to carry, so compact that it does not require much space for storage, safe, and stable, and ensures reliability in water imperviousness.

This lightweight, water imperviousness, and stable water tank can be implemented by combining a folding waterproof sheet tank 1 which is unstable in formation in spite of the expected complete water imperviousness and weight reduction, with several divided walls 2 which are stable in formation. When the water pressure is unilaterally imposed, a spacer 4 provided on one side of the inner surface of each divided wall 2 works to maintain the clearance 10 between divided walls 2. After use, all divided walls 2 are removed one after another, and the waterproof sheet tank 1 is made inclined to empty the water out. Since all individual parts are light and small enough to be carried by one person, it is possible to make this stable water tank instantly in any place, and therefore, it has various uses including as a swimming pool for children, an outdoor bathtub for family use, and a quickly-made water reservoir in an emergency.