PURPOSE:To simplify operations as well as raise the safety of operations by a method in which an early-period operation space is formed under a condition that the bottom of caisson is not pressurized, and excavation operation is made in the space under a condition that the bottom of the caisson is pressurized to form a wider operation space than the early-period operation space. CONSTITUTION:A caisson 1 with a box-like concrete caisson body 2 is settled down into the seabed ground 9 and a blade 2a is formed on the lower part of the caisson body 2. A discharge pipe 6 is provided to a material lock 5 provided to the caisson 2 under a condition that a suction port 6a is connected with the bottom 2b of the caisson and a discharge port 6b is extended to the outside of the caisson 1. Under a condition that the bottom 2b of the caisson is not pressurized, air is supplied from the surrounding of the pipe 6 into the bottom 2b of the caisson. When suctional action is started through the pipe 6, sand 10a is discharged through the pipe 6 to form an early-period operation space 11 near the face 2d of the bottom 2b, and as excavation proceeds, an operation space 12 is formed.