To provide a cofferdam construction method capable of easily constructing, whithout using any special apparatus, structural body for forming a working environment in dry condition around piers.

The cofferdam construction method comprises a process A for connecting a plurality of cofferdam members 4 together in annular form at a predetermined site while setting them afloat on water, each of the members having an exterior wall part, an interior wall part and a cavity part; a process B for completing the cofferdam structural body by stacking cofferdam structure 4, other than cofferdam annular members 4, each opened on its sides to be abutted against the adjacent cofferdam members 4 to construct a cylindrical structural body 6 and at the same time injecting water into the cavity parts of the cofferdam structures 4 constituting the cylindrical structural body 6, then adjusting the upper end of the cylindrical structural body 6 so that it is always located at an appropriate height above the surface of water 15; and stopping the stacking work of cofferdam members 4 when the cylindrical structural body 6 is finally grounded in the water; and a process C for draining water preset in an area surrounded by the cofferdam structural body.