To ensure high water cut-off performance without being influenced caused by the inclination, unevenness and the like of the top face of a footing and an external force caused by a water flow and a water level difference.

A temporary cofferdam comprises a cofferdam body surrounding the outer circumference of an existing underwater structure, a reinforcing ring integrally provided at the bottom end of the cofferdam body and an elastic seal member installed along the circumference of the reinforcing ring. The reinforcing ring is spaced apart from the top face of the footing of the existing underwater structure. The clearance between the reinforcing ring integrally provided at the bottom end of the cofferdam body and the top face of the footing of the existing underwater structure is closed with underwater concrete placed on the top face of the footing up to the height burying at least the seal member of the reinforcing ring for the purpose of water cut-off.