Abstract: A sand gathering system has a submerged sand pump capable of sucking slurry such as sand and gravel from the bottom of sea, lake, pond or the like, a sedimentation tank disposed at the discharge opening of the suction pump, and a discharge tank for discharging the muddy water separated from the sand slurry in the sedimentation tank. This system is operated such that the muddy water separated from the slurry is discharged to a region near the bottom of the sea, so that the muddy water is not diffused to the surface region of the sea water. To this end, the sand gathering system has an apparatus for preventing diffusion of the muddy water. The apparatus comprises a discharge pump connected to the discharge portion of the discharge tank and a discharge pipe connected to the discharge opening of the discharge pump and leading to a region near the bottom of the sea, lake, pond or the like.