PURPOSE: To reduce a cost, by a method wherein, after filling sand, having the same weight as that of upper concrete placed on the deck of a caisson, is charged in a hull type caisson, the caisson is sunk so that the upper concrete is submerged.

CONSTITUTION: The inside of a center tank 16 and wing tanks 17 and 18 of a hull type caisson 11 is filled with a filling sand 19 for balance having the same weight as that of upper concrete intended to be placed on a deck 12. Thereafter, upper concrete 20 and breakwater concrete 21 are placed on the deck 12. Sea water is then poured into the caisson 11 through openings 13 of the deck 12 by means of openings 22 of the upper concrete 20 to seat the caisson 11 on rubble- mound 23 so that the upper concrete 20 is submerged down to lower than an average high tide waterline H.W.L. After the filling sand 19 is further charged through the openings 22 and 13 to replace sea water in the caisson 11 with the filling sand, underwater concrete 20a is placed in the openings 22.