To further enhance the level of water cut-off by an elastic member when the periphery of an underwater structure is brought into a waterless dry working environment.

This structure for a cofferdam comprises a working caisson 16 having one or more steel shells and the elastic seal member 24 attached to the bottom of the working caisson 16. A lateral deformation prevention member 26 for preventing the elastic seal member 24 from being deformed inward of the working caisson 16 is disposed on at least a part of the inner periphery of the elastic seal member 24. Even if the elastic member 24 hydraulically receives a horizontal force, the fixing water cut-off function of the elastic seal member is not reduced.