To effectively reduce the drag in various motions of a flying object.

The thrust T of an aircraft is estimated by a thrust estimating means M1, the motion (the speed, the angular velocity, the angle of attitude, and the angle of attack) of the aircraft is detected by a motion detecting means M, and then, a drag estimating means M3 estimates the drag D of the aircraft based on the thrust T and the motion. An operation calculating means M4 changes the rudder angle of a flap by an operating means M5 by a small quantity, and monitors the increase/decrease in the drag generated as a result, and further repeats the operation of changing the rudder angle by the small quantity to converge the rudder angle to a target value at which the drag of the aircraft is minimized. Since the drag reduction is controlled by monitoring the actual drag, it is extremely effective, and the effect can be demonstrated irrespective of the motion of the aircraft.