To provide an apparatus related to high-density dredging that dredges mud in a wide range without making an impact exerted on the mud or dispersing the mud in water.

By the structure of the apparatus, a mud collecting box 16 is arranged at a vertically lower location of a floating body 17. The mud collecting box has an almost U-shaped skeleton with an openable door 19 vertically movably fitted therein on the side of an opening, and has a hermetically sealing cover 18 on an upper surface in a manner of vertically movably being fit into the box. The mud collecting box 16 has a suction port 20 formed therein which is connected to a suction pipe 6. The mud collecting box 16 has an open bottom surface and wheels 22 mounted thereon. Then tow ropes 3, 4 connected to a tow ship 2 are linked to both frames of the opening of the mud collecting box 16, and lifting lugs 29 for hanging the apparatus by means of a crane or the like are attached to both the frames.