To provide a dam forming raw material for water cut-off instead of a sandbag.

A water bag 1 of forming a plurality of strips of recessed places 2a, is formed by injecting tap water or water being a water cut-off object into a bag body 2 via a water injection hole 3. This water bag is stacked so that a projecting part of the other water bag is positioned in the recessed places. A dam is formed by engaging the mutual water bags 1, and water is cut off by this dam. After attaining the purpose, inside water is discharged by opening the water injection hole 3, and is prepared for the next use.