To assemble and disassemble a cofferdam construction simply and quickly.

This structure 18 is divided along the vertical direction, actually constituted of two box body blocks 20 of the same construction as main structural members, and the block 20 is provided with inside and outside plates 20a, 20b, a bottom plate, and a pair of end plates 20e. The block 20 is constructed so that the lower end side of the, inside and outside plates are blocked with the bottom plate, a pair of openings of a U-shaped part are respectively blocked with the end plates 20e, and the upper end side only is opened into a box shape. The block 20 is manufactured in a manufacturing yard such as a dock nearby the repairing field of a bridge, towed by a tug boat, and carried to the vicinity of the bridge. The carried box body blocks 20 are connectedly assembled with a bridge pier 12 as the center so that box body connecting packings 20k are sandwiched between the opposite end plates 20e, and joint tools 20i are fitted between brackets 20i and fastened, and when the assembly is completed, they are sunk and provided on a footing. An installation packing is interposed on the underside of the bottom plates of the blocks 20.