To enhance the bending rigidity of a bottom board, prevent a bending crack from being caused and prevent the bottom board from being separated from a frame body.

A steel panel 6 is submerged in a body of water 2 (102), and underwater concrete is laid in the body to construct the bottom board 34 on the bottom 3 (104). The board 34 is constructed so as to embed in the underwater concrete a strut 36 on the lowermost stage installed within the steel panel 6. Then, a dry-up process is executed (step 105), and a bridge pier 1 is constructed on the board 34 (106). After the construction of the pier 1 is completed, water is poured into a working space 51 (step 107), the steel panel 6 is separated right above the board 34, and an upper frame body 6a together with the strut 35 on the uppermost stage is removed (108).