PURPOSE: To efficiently keep the inner pressure at the normal value by starting an inner pressure adjusting fan in addition to a normal fan when the inner pressure of an air dome is abnormally reduced, and stopping the inner pressure adjusting fan when the inner pressure is returned to the normal value.

CONSTITUTION: When the inner pressure of an air dome is abnormally reduced, the first pressure switch 11a is operated, and the second pressure switch 11b and a transfer switch 12 are operated in sequence. When a relay 14 is excited and a contact r1b is closed, the excitation of an inner pressure adjusting fan is started, and the inner pressure adjusting fan is started. When air is fed by a normal fan and the inner pressure adjusting fan, the inner pressure is quickly increased. When the inner pressure is returned to the normal value, the switch 11b is operated, the relay 14 is de-excited, the excitation of the inner pressure adjusting fan is cut off, and the fan is stopped.