PURPOSE: To regulate a grain sieving effect at a velocity of an ascending stream by passing the sand stream of sand mixed with dust on the horizontal or slightly inclined surface of a sieving bed for particles heavier than sand particles and using the ascending stream as a carrier stream for the dust components.

CONSTITUTION: An air permeable intermediate plate or temporary bottom 16 having a transverse direction grid 17 and a longitudinal direction grid is fixed spatially from a bottom plate 12 to a housing wall body in a housing 10 and the sieving bed 18 for pellets or balls of copper or steel is placed on the intermediate plate 16. Air is introduced in an arrow X direction by an air feed conduit 40 having a throttling member 38 into the bottom space 20 of the housing and is blown through the sieving bed 18. The intermediate plate 16 is connected to a knocking mechanism. The air flows through a sand-dust bed 36 and entrains the dust toward the air groove holes 24 of a guide pipe 26. The dust is transported by the air (arrow Z) of the guide pipe 26. The mixture composed of the sand and dust introduced by a sand introducing means 30 is separated on the sieving bed 18 and the sand is discharged from a sand discharge port 34.