To provide a cofferdam construction method at a great application water depth, and to reduce a work burden on a worker, and to shorten a construction period, by reducing diving operation steps, in the cofferdam construction method.

A cut-off structure 30 has a constitution in which first and second annular elastic bodies 32 and 33 are arranged in a bolt-connected state in a supporting frame 31 arranged at the lower end of a cut-off wall 11. The first and second annular elastic bodies 32 and 33 in the state of being pressed against an anchoring surface of a footing 51 are deformed in such a shape that lower ends are crushed, so as to be brought into close contact with the anchoring surface of the footing 51. An injection pipe 14 communicating with a section from an upper end of the cut-off wall 11 to a lower end thereof is provided inside the cut-off wall 11. A filler is injected into a space 34 between the first and second annular elastic bodies 32 and 33 of the cut-off structure 30 on the lower end side of the cut-off wall 11, from the upper end side of the cut-off wall 11 via the injection pipe 14.