PURPOSE: To prevent the pollution of river water by a method in which a hole is provided at the center of a hull, the lower part of a sheathing frame is inserted through the hole into a bottom ground of the water, and the excavation of the bottom ground is made under the condition that water is isolated.

CONSTITUTION: A hole 2 is provided in the center of a hull, and a working base 3 is provided in the hole 2 in a vertically movable manner. A hole 4 through which a sheathing frame is vertically moved is provided to the working base 3. The sheathing frame is provided in the hole 4 in a vertically movable manner, a working base holder 6 to vertically operate the working base 3 is provided to the hull 1, and a sheathing frame elevator 23 to push the lower part of the sheathing frame being vertically operated into the bottom ground of the water is provided to the base 3. An excavator A is provided on the base 3, and a water supply and discharging device is provided to the hull 1. Since the bottom ground of the water can be excavated while pushing the lower part of the sheathing frame into the bottom and discharging water in the frame, the excavation work can be made under the condition that water is isolated and the pollution of river water can be prevented.