To reduce costs and shorten a construction period by eliminating a limit to a difference between the size of a footing and that of a bridge pier.

A temporary cofferdam 12 is composed of a cofferdam wall body 18 which is assembled in such an annular shape as to have an inside diameter larger than a diameter of the footing 14, and an annular form body 20 which is locked to an outer periphery of a lower end of the wall body 18. The cofferdam 12 is sunk in the ground at the bottom of water outside the footing 14. After the completion of timbering of the wall body 18, a foot protection concrete part 26 is formed. The concrete part 26 is formed by placing underwater concrete in the form body 20; a lower end side of the wall body 18 is partially embedded in the concrete part 26; at the completion of that, water is discharged from inside the cofferdam 12 so that the cofferdam 12 can be put into a dry state; and repair work such as the repair and strengthening of a bridge pier part 16 of an underwater structure 10 is performed. At the completion of the work, the whole of the cofferdam 12 is removed with the concrete part 26.