An object is to provide a closing member for an opening, the closing member capable of more reliably maintaining a lightning-protective function. In addition to a first insulator 63, a second insulator 65 is provided between a door body 30 and a wing inner surface 11b around an opening 12. When the first insulator 63 falls off, the second insulator 65 becomes interposed between an inner peripheral planar part 64 and the wing inner surface 11b. With the first insulator 63 being tightly interposed between an outer peripheral planar part 62 and the wing inner surface 11b, the second insulator 65 faces the wing inner surface 11b with a predetermined clearance C therebetween. The outer peripheral planar part 62 is formed at a position further away from the wing inner surface 11b than an apical surface 65a of the second insulator 65 mounted on the inner peripheral planar part 64.