To conduct operation under the same state as a land even in a great water depth section by sinking a temporary coffering caisson body, in which one side is open, on the foundation of the lower end section of a structural section, draining the inside of the caisson body and contact-bonding the caisson body with a structure.

A temporary coffering caisson body, in which a plurality of caisson body units 11, 11a-11c, in which each one side is open, are assembled vertically and a plurality of compartments are formed inside the units 11, 11a-11c by parting covers, is floated in the sea 1. One tips of drawing wires 4 passed through blocks 3, 5 installed on the structure 2 side are connected at the proper places of the temporary coffering caisson body. The temporary coffering caisson body is pulled by the drawing wires 4, and sunk on a foundation 10 formed at the lower end section of a structure 2. The temporary coffering caisson body is contact-bonded with the structure 2 by draining water on the inside of the caisson body, and operation is conducted under the same state as a land in the caisson body.