To reduce an influence of an external force and manufacturing costs and enable reuse, by reducing the outside dimensions of a working caisson.

The working caisson 10, which is composed of an annular steel shell formed by preassembling a plurality of stiffening-plate-structure pressure-resistant blocks 12, is floated and towed to a construction site in a state wherein a detachable buoyancy body 40 is mounted on the outer periphery of the working caisson 10, and installed in such a manner as to enclose a working object 8. After the removal of at least a part of the buoyancy body, the inside of the working caisson is drained, so that the working object can be fixed by a difference between the water pressure of the inside of the steel shell and that of the outside thereof, and so that a working space can be formed.