PURPOSE: To exhibit the performance of an airplane at maximum and use it safely and efficiently by controlling the flap angle based on the speed of airplane, and maintaining the flap angle at optimum value at take-off running and the like of the airplane.

CONSTITUTION: A speed signal of an airplane detected by a speed system 14 and a flap angle signal detected with a flap angle potentiometer 21 are respectively input to a computer 16 to compute and process. Meanwhile, the control signal to obtain an optimum flap angle for the speed of an airplane is transmitted from the computer 16, and input to a hydraulic device 18 to actuate it. An actuator 20 is actuated to perform change of flap angle 3. Hereby, optimum flap angles against the speed of the airplane at take-off, landing, and the like are instituted, respectively adjusted so as to be maximum increase of lift by flap and minimum increase of resistance by flap, and performance of the airplane is exhibited at maximum.