To sand the sea bottom broadly by swing motions, accurately to uniform thickness, while restricting seawater pollution, at proper efficiency and without requiring a complex sanding mechanism, power or special technique.

This sea bottom sanding device is provided with a discharge pipe 4 connected to a pump to discharge the sand slurried, ladder device 5 to vertically move a sanding head 12, an expanded pipe 10, which is connected to the discharge pipe 4 and supported on the ladder device 5 and has rectangular cross sections which changes the flow downward on the tip side and gradually expands, and the sanding head 12 connected to the expanded pipe 10. Then, this sanding head 12 is spirally twisted while being gradually expanded so that the discharge port 12a of the sanding head 12 in the longitudinal direction faces and has a large width in the longitudinal direction of the ship, allowing effective and uniform sanding utilizing the swinging of the ship.