To provide an economical cofferdam structure and a cofferdam construction method.

The cofferdam structure 18 is provided with a plurality of precast panels 20 of plate shape separably connected through cut-off members 21 interposed between the end faces, and installed to surround the outer periphery of an underwater structure such as a bridge foundation. The panel 20 is provided with inclined end faces 20a with a predetermined angle provided at both ends in a cross direction, a semicircular recessed part 20b vertically extended almost at the center of the end faces, and brackets 20c provided at the side faces of the panel 20. The cut-off member 21 encloses a cylindrical pipe 21a inside, and has a cylindrical part 210b inserted in the recessed part 20b, and fin parts 211b projected from the outer periphery of the cylindrical part 210b. Each fin part 211b is interposed between the inclined end faces 20a and clamped between the inclined end faces 20a by mounting a fastening bolt 28 to the brackets 20c.