To provide an environmentally harmless and economical cofferdam structure and a cofferdam closure method not attended with mass dredging.

In the cofferdam structure for surveying, repairing or reinforcing an underwater existing structure, reinforcing ring bodies of predetermined heights formed by circumferentially connecting a plurality of divided cofferdam steel plates at the outer periphery of the existing structure, and shaped to form a working space with a predetermined space at the outer periphery of the existing structure, are pressed into the water bottom ground around the existing structure while being connected in a plurality of stages in a depth direction, and a cofferdam member extending to the water surface is installed on the reinforcing ring bodies to form a dry working space around the outer periphery of the existing structure.