To quickly exhaust dust and to efficiently operate an exhaust fan by optimally controlling the exhaust fan and exhaust dampers disposed respectively in a plurality of dust generation areas.

A control device 8 detects the amount of dust in the dust generation areas DR1, DR2, increases a rotational frequency of the exhaust fan 5 when the detected amounts of dust in all of the dust generation areas DR1, DR2 are at high levels, decreases the rotational frequency of the exhaust fan 5 when the detected amounts of dust in all of the dust generation areas DR1, DR2 are at low levels, further increases openings of the dampers 3A, 3B when the dust generation area DR1, DR2 where the detected amount of dust is at a high level, exists, and openings of the exhaust dampers 3A, 3B in the area is not 100%, and decreases the opening of the exhaust damper 3A, 3B of the other dust generation area DR1, DR2 where the detected amount of dust is not at a high level, when the dust generation area DR1, DR2 of which the detected amount of dust is at a high level, exists, and the opening of the exhaust damper 3A, 3B of the area is 100%.