PURPOSE: To enable to perform an efficient dredging work, by a method wherein dredged mud is selected and crushed to produce ready mixed concrete-like fluid, and the fluid is processed by means of plural tanks and guide pipes.

CONSTITUTION: A screw conveyor, to which a spiral fin 47 is forwardly and reversely rotatably mounted, is mounted in a horizontal casing 46 with both closed ends to form a hopper 45. Dredged mud, conveyed by a second screw conveyor 29, is dropped and charged through a hopper part 51, the mud is conveyed alternately to either a first tank 31 or a second tank 32 through plural guide pipes 49 or 50, and is forcibly fed and discharged through discharge pipes 40 or 40' from the tank 31 or 32. This enables increasing of an amount of fluid mud processed, resulting in the possibility to perform an efficient dredging work.