The present invention is directed to reducing noise resulting from a wing tip vortex of a wing such as a flap. A flap of an aircraft includes a flap main body provided to a parent wing in a deployable manner, and a vortex generation portion including a protruding member, the protruding member being provided to protrude from an upper surface at least at an end of the flap main body on an outboard side in a span direction and forming a predetermined angle to an axial direction of the aircraft. A rear end of the protruding member is located on the side adjacent to a side-end edge at the end of the flap main body in the span direction relative to a virtual line passing through a front end of the protruding member and being parallel to the axial direction.