To control the position of a transition point of a boundary layer in order to effectively reduce the drag in various motions of a moving body such as an aircraft.

A transition point detecting means M1 detects the position of a transition point (t) from a laminar flow boundary layer BLL to a turbulence flow boundary layer BLT based on the output of a sensor S such as a hot film arranged on a surface of a main wing W of an aircraft. A control means M2 changes the rudder angle δf of a flap F by a small quantity, and monitors the movement of the transition point (t) generated as a result, and further repeats the operation of changing the rudder angle δf by a small quantity, and moves the position of the transition point (t) as backward as possible to reduce the drag. Since the drag reduction is controlled by monitoring the actual position of the transition point (t), it is extremely effective, and the effect can be demonstrated irrespective of the motion of the aircraft.