An air-conditioning duct structure of an aircraft includes an air-conditioning duct that is provided on each of a starboard side and a port side and guides conditioned air to a vicinity of a ceiling of a cabin in the aircraft. The air-conditioning duct includes a blowout portion and a blowout upstream portion. The blowout portion blows out the conditioned air to the cabin from an outboard side of the ceiling to an inboard side of the ceiling and toward a lower part than the ceiling, and the blowout upstream portion communicates with an upstream side of the blowout portion. In addition, the blowout portion or the blowout upstream portion is curved to cause a terminal end side of the blowout portion to be inclined downward with respect to a horizontal direction.