PURPOSE: To obtain enough mechanical strength and low FST value by arranging the first fire barrier membrane between the first and the second honeycomb core layers and arranging the second and the third fire barrier membranes on the outside of the first and the second honeycomb core layers.

CONSTITUTION: Two honeycomb core layers 2 and 3 are separated by the center septum layer 5 and the surface skins 4 and 6 are respectively provided in front and back of a sandwich panel 1. The center septum layer 5 and the surface skins 4 and 6 are respectively fire barrier membranes and in addition, membranes 7, 8, 9 and 10 are pref. prepregs such as low FST phenolic resin prepregs. The fire barrier membranes 4, 5 and 6 are made of many different materials contg. both an inorg. fiber membrane or an inorg. paper and a vermiculite film. The inorg. paper is used for one of the surface skin or the septum layer with a thickness of 0.254-0.889 mm. The film thickness of the vermiculite mica film barrier is 0.127-0.762 mm.