PURPOSE: To prevent air stream exfoliation generated in an upper surface of aircraft main wings.

CONSTITUTION: An air blowout port (slit 3) provided in a rear half part of main wing upper surfaces, air intake port provided in a lower surface of a fuselage and a duct 5 for connecting the air blowout port to the air intake port are provided, and air is taken in from the fuselage lower surface of high pressure and blown out to the main wing upper surface from the air blowout port toward the rear to prevent an air stream from its exfoliation.