A base wing 11, as attached at its root end portion to the rotor head of a rotational drive unit, is provided with a front wing 12 and a rear wing 13 having substantially equal spans. The vortex, as generated at the wing tip, is divided into relatively weak tip vortexes of a front wing vortex 12c generated at the tip of the front wing and a rear wing vortex 13c generated at the tip of the rear wing 13. Since the front wing 12 and the rear wing 13 have equal spans, on the other hand, the front wing vortex 12c and the rear wing vortex 13c are close to each other so that they interact positively with each other and are weakened and diffused. As a result, while the helicopter is hovering, the interactions between the retreating rotor blade of the rotor and the tip vortex generated by the tip of the advancing rotor blade are drastically reduced to suppress the occurrence of the BVI noises.