An aircraft including: a penetration member that penetrates an airframe between an inside and an outside via an opening provided in the airframe; a seal that seals a gap set between an opening formation member forming the opening and the penetration member; and a retainer that presses the seal against the penetration member and the opening formation member, wherein each of the penetration member and the opening formation member includes a receiving section that receives the seal, both of the receiving sections are arranged along a direction connecting one side and the other side of the gap with the gap therebetween, and the seal is fixed to only one of the receiving section of the penetration member and the receiving section of the opening formation member.