PURPOSE: To lead effectively dusts turning on a whirl in a casing and improve the discharge efficiency of separated dusts by making an oblong dust discharge opening on the casing along the axial direction thereof.

CONSTITUTION: Fresh air introduced from an inlet 2 is forcibly whirled in a whirl passage. Dusts turned along the inner side of a casing 1 flow into a dust discharge hole 11 due to an inertia force. The dusts after the hole 11 further flow to the forward end of the dust guide tube 13 of a dust collection device 12 due to an inertia force, and are temporarily held in a dust discharge valve 14. When the inside of the casing 1 is exposed to positive pressure due to intake air pulsation pressure or to a vibration, the slit of the dust discharge valve 14 opens automatically, thereby discharging the dusts outside.