PURPOSE: To promote work efficiency by floating a barge in an excavated tunnel filled with water, excavating the excavated tunnel by means of a dredging machine mounted on the barge, and absorbing/moving mixture mixing excavated earth with water through a slurry pipe.

CONSTITUTION: An excavated tunnel 2 formed by primary excavation is filled with water by certain depth. After that, a barge 3 is floated on the water, at the same time, one end of each of wires 5 for moving the barge is borne on each of four corners of the barge 3 in a windable manner, and the other end of each of the wires 5 is anchored to an earth retaining dike I side. Then, winding of the wires 5 is controlled by commands from an operation room 12, and the barge 3 is moved to a required work position. In addition, a boom 11 is operated, an absorption type dredging machine 4 with a cutter is operated to absorbe excavated earth with a pump, and it is conveyed to an earth separator 9 on the ground through a slurry pipe 6.