To provide a lightweight and simply structured flood damage preventing device which can be rapidly installed at a predetermined position in a short time and with less labor even when a flood comes, and can surely prevent immersion.

The flood damage preventing device comprises; a front face structural body A including a cut-off wall part 2 having an inclined surface which gradually rises toward the rear side, a water-pressure reinforcing part 3 supporting the cut-off wall part 2 from the rear side, a base part 4 supporting a lower edge part of the cut-off wall part 2 and a lower edge part of the water-pressure reinforcing part 3, and a cut-off sealing part 5 having outside areas 5b, 5b preventing a water leak by being pressed against the installation surface to be deformed for sealing and extending to both sides of the base part 4; side part structural bodies B1, B2 arranged on the outside areas 5b, 5b of which each of the side part structural bodies B1, B2 includes a groove part 6c slidingly engaged with a side edge part 2e of the cut-off wall part 2, a side wall cut-off reinforcing part 7 preventing the water leak from the sides, and a side wall cut-off sealing part 8 preventing the water leak by being pressed against the side wall to be deformed for sealing; and a fixing means 4a fixing the front face structural body A to the installation surface.