PURPOSE: To deposit oily dirt onto the inside surfaces of a filter and thereby remove the oily soot efficiently by a method wherein polluted air containing oil dust at a kitchen ventilator is passed through a grease filter.

CONSTITUTION: When the polluted air containing the oil soot is discharged by a fan 8, the air passes through the grease filter 11, wherein the air sucked in through a front surface 18 of the filter 11 is throllted at opening parts 19 of the filter 11 and is deflected when passing to the back side of the filter 11. When the polluted air is throttled and deflected, the oily dirt is deposited onto the inside surfaces 21 of the filter 11, so that the air passed through the filter 11 to the back side thereof contains only a smaller amount of oily dirt. Accordingly, since the amount of oily dirt penetrating into the interior of the ventilator is reduced, the ventilator undergoes a smaller reduction in its performance.