To provide a light impervious device capable of facilitating the conveyance/storage, easily and quickly carrying out the construction of an impervious wall and automatically increasing weight with water to become the impervious wall.

A supply system water absorption sheet 8 sealing a high- molecular water absorbing agent having an injection port 8b is mounted on the upper surface of the bottom plate side of an L-shaped foldable impervious plate 7 at a hinge 7a, and water permeable self-supply system water absorption sheets 9 are mounted to the inside of a vertical plate part. The impervious plate is placed in parallel in the front of the lower end of a shutter for a garage. The water absorption sheets 8 and 9 are expanded from 100 to 200 times by water of floods or water of water supply to maintain the sheets in an L-shape and, at the same time, they become heavy to form an impervious wall as well as a sand-bag.