To stabilize a vortex generated from the leading edge by forming a triangle or a trapezoid so that the chord length may be increased from the upstream to the slip stream, and forming a section so that a part on the leading edge side from a bent line drawn linearly from the upstream to the slip stream may be bent upward at a designated angle.

The whole plane form is such a triangle that the chord length is increased from the upstream end pivoted on the leading edge 3 of a belly fitting part of a sweetback wing 2 toward the slip stream end pivoted on the leading edge 3 positioned substantially half the wing width, and divided into four parts by a dividing line intersecting perpendicularly to the leading edge 3. Among the above, the uppermost stream side one has a triangular plane form, and the slip stream side remaining three parts are trapezoidal so that the chord length is enlarged toward the slip stream side. A bent line 6 passing the position with a fixed proportion of the chord length in each wing width position is provided like a straight line from the upstream side to the slip stream side. The proportion is fixed in such a manner that the bent line 6 is positioned on the flap leading edge 9 side at least from the center of the chord length of each position.