PURPOSE: To reduce the thermal deformation in an outer space dividing a base plate and a dielectric core constituting the 1st sandwich panel of double sandwich structure into plural pieces.

CONSTITUTION: A ground conductor 7 is formed at one side of a nylon fiber reinforced dielectric honey-comb core 6 by a carbon fiber reinforced plastic, a base plate 5 is formed on the other side and the 1st sandwich panel A is constituted by coating a metallic foil to the base plate 5 so as to form a radiation element 4. In order to improve the entire rigidity of the antenna panel 1, the double sandwich structure is constituted by combining a light weight aluminum honey-comb core 8 and the 2nd sandwich panel B consisting of a skin member 9 made of a carbon fiber reinforced plastic. The base plate 5 and the core 6 are cut off to divide the panel A into eight pieces, allowing to release a force causing out-of-plane deformation due to temperature difference, reduce thermal deformation and make the entire panel flat.