To provide the maximum safety in relation to corrosion and pollution with the limited maintenance by mixing a steam flow and a high-temperature air secondary flow in an outlet of a boiler, and filling the high-temperature air secondary flow including the steam into heating and air-conditioning circuit of aircraft cabin.

When a humidifying device 1 is started, open of a supplying valve 9 and a partitioning valve 8 is commanded by a control unit 13, and these valves are opened till the charging high water level of a boiler 6 is detected by a water level sensor 12. Through the whole of the operating time, open position of a high-temperature air flow adjusting valve 2 is adjusted so as to maintain a constant dew condensation point in an aircraft cabin. On the other hand, when a low water level is detected by the water level sensor 12, the control unit 13 commands the open of the supplying valve 9 and the partitioning valve 8 till a high water level is detected. Periodical supply of water to the boiler is secured by this work.