PURPOSE: To eliminate a preliminary device for a feed air fan and to prevent damage of function even during the stop of the feed air fan, according to a method wherein a bypass line is installed to the feed air fan and an open air processing device.

CONSTITUTION: An open air processing device 2, a bypass duct 10, bypassing a feed air fan 3, and a main exhaust gas damper 12, interlocking with the stop of the feed air fan 3, are provided. During the stop of the feed air fan 3, the open air passes through the bypass duct 10, is guided in a building 1 directly through a feed air duct 4, and is emitted out of an exhaust cylinder 8 by means of an exhaust gas duct 5 and an exhaust gas processing device 6 with the aid of an exhaust fan 7. In which case, a main exhaust damper 12 receives a feed air fan stop signal 11, and is automatically controlled to a determined opening. With this state, operation of the exhaust fan 7 alone is continued. During the stop of the feed air fan 3, the main exhaust damper 12 is throttled to a set opening to perform operation, and this enables prevention of production of an abnormal negative pressure in the building 1.