To provide a flat water stop material that need not be placed on top of another, but can easily be connected laterally with another for achieving water stop conditions, and also its installation structure.

This is a water stop material 1 of which one side surface is a layer 2 made of an adhesive water stop material and the other side surface is a layer 3 made of a non-adhesive water stop material. The adhesive water stop material layer 2 has a thickness of more than 2 mm. These flat adhesive water stop materials 1, 1 are laterally brought into contact with each other in their side surfaces so that their layers 2, 2 of adhesive water stop material may adhere to each other, achieving continuous water stop conditions.