PURPOSE: To perform a compressing operation of a lump of dust automatically, in a short time and easily with a simple operation of only opening and closing solenoid valves, by compressing and stowing the lump of dust with an air flow.

CONSTITUTION: When the rear end Ae of a flexible hose is pulled out from a spigot B after finishing a stowing operation of dust (a), solenoid valve I and C are closed and H and J are opened simultaneously with said pulling. Then, only for a short time a blower M continuing to rotate without changing its rotational direction produces an air flow, which compresses a bulky and large volume lump of dust (a) from its upper side to make the volume of the lump small and flows out of a dust container F and a lower can E and is filtered by a secondary filter K, and the clean air is discharged. Next, simultaneously with the stop of the blower M, the valves H and J are closed and the valves I and C are opened, and the air flows in the directions of arrows R1, R2 so that it is prepared to be used as a sucking air for stowing the dust when the next dust collection is started.