There is provided a drainage system of an aircraft which discharges moisture having entered inside horizontal tails, which are structures of an airframe of an aircraft, toward the rear end of a fuselage, the drainage system including: a drainage channel which includes an introduction end and a discharge end, and by a capillary action delivers moisture introduced from the introduction end toward the discharge end; and a heater which heats and dries the moisture delivered through the drainage channel. The drainage channel of the present invention is preferably composed of a paper drainage material. It is also preferable that the introduction end of the drainage channel is provided in a lowermost region in the vertical direction within an area in which moisture can accumulate inside the horizontal tails while the aircraft is parked.