PURPOSE: To enable the successive conveyance of sludge from a river bed and the sufficient mixing of the sludge and a solidificating material to be carried out, by using the first jet pump in which the mixed liquid of water and the solidificating material is used for a driving fluid and the second jet pump in which the mixed liquid of the water and a sedimenting agent is used for a driving fluid.

CONSTITUTION: The inlet opening of the first jet pump 1 is inserted to a sludge layer so as to absorb sludge 12, next the absorbed sludge is discharged into a settling tank 3 by the second jet pump 2. As this result, the first jet pump 1 uses the mixed liquid of water 4 and a solidificating material 5 for a driving fluid as well as the sludge can be discharged successively from a river bed by the jet pumps, so that the sludge 12 and the driving fluid are mixed powerfully each other in a carrying pipe, and the sludge 12 and the solidificating material 5 can be sedimented to the settling tank 3 in a sufficiently mixed condition. And as the second jet pump 2 uses the water 4 and a sedimenting agent 22 for a driving fluid, they are sufficiently mixed, and a sedimenting function can be effectively exhibited. Moreover, as the supernatant liquid inside of the settling tank 3 is utilized for the water of the driving fluid of both pumps, the water in the spot can be effectively utilized.