PURPOSE: To improve in durability of elements and to reduce aerodynamic drag by providing radio permeable materials at least at the lower portion of the outer wall of a main body the inside of which is filled with gas for floating and the elements for receiving microwaves as an energy source in the main body.

CONSTITUTION: A flying body 10 is filled with gas 12 for floating inside the main body. In this instance, Rectennas 13 as energy receiving elements are disposed on the ceiling portion inside the main body. An envelope 11 constituting the outer wall of the main body is made from electric permeable materials 14 at least its lower surface. Thus, maicrowaves may be satisfactorily received by means of the Rectennas 13 so as to eliminate troubles to transmit maicrowaves. For electrical wave permeable materials 14, for example, coated clothes or plastic films are used. Since there is no Rectenna 13 or the like on the outer surface of the envelope 13, aerodynamic drag is reduced.