To provide a water cut off device for an opening capable of constructing a weir for closing the opening of a structure with an easy operation while having a high water cut off capacity.

The water cut off device 10 for an opening is used for constructing the weir 50 for preventing fluid from flowing in through the opening 3. The weir is composed of a tube body 11 of an elastic body to be bulged and expanded by the fluid poured into the body and a support means 20 for positioning and supporting the tube body 11 between columns 4, 5 or walls forming the opening 3 of the structure.