PURPOSE: To shorten the term of construction and need no sea-bottom dredging after a cofferdam is removed, by a method wherein a ballast can be contained in a floating body, and wherein piles are housed on the side sections of the floating body, and wherein the cofferdam is constructed to be hung from the piles.

CONSTITUTION: A floating body 1 is taken in tow to the site of a cofferdam, and a ballast is contained in the floating body 1 there, and the floating body 1 is sunk. After that, piles 2, 2a are supported by the side sections of the floating body 1, and at the same time, the piles are driven to the sea-bottom, and the floating body 1 is served as a supporting guide when the piles are driven. When removed, the piles 2, 2a are respectively taken out of the ends in order, and after the work for taking out is completed, the floating body 1 is made to float and is moved to the following construction site. As a result, the construction is facilitated, and the term of the construction is shortened, and the removal can be facilitated, and so the piles can be used repeatedly.