PURPOSE: To easily drop not only fine material but coarse material as well and to homogeneously distribute the fine material and the coarse material by providing the circumference of a rotary drum having a casing of a perforated sheet metal with plural door paths.

CONSTITUTION: The drum l is formed of the sheet metal 5 having holes 6. This casing has the plural door paths 60. When the drum l rotates counterclockwise, the door paths 60 are closed by a door 61 connected by a hinge shaft 62 but when the drum rotates clockwise, the door paths 60 are opened and the coarse material falls and, therefore, whether the coarse material is removed or only the fine material is sieved is determined according to the rotating directions of the drum 1. The fine material or the coarse material falling from the casing of the drum 1 is carried from a belt conveyor 22 through an outlet side elevator wheel 26 to a silo 3. When the mixing is needed again, the belt conveyor 22 is rotated backward and the material is charged through a terminal device 65 and an inlet side elevator wheel 26 into the drum 1 where the mixing is repeated.