To provide a cleaning method for an air conditioning system with a simple structure, capable of inexpensively and positively renewing refrigerating machine oil, and to provide an outdoor unit used therein.

The air conditioning system 1 is provided with the outdoor unit 3 having a compressor 11, an indoor unit 5 provided with an indoor expansion valve 51 and an indoor heat exchanger 53, a liquid tube 7 connecting the outdoor unit 3 to the indoor expansion valve 51 of the indoor unit 5, and a gas tube 9 connecting the outdoor unit 3 to the indoor heat exchanger 53 of the indoor unit 5, and the cleaning method is for cleaning at least the liquid tube 7 and the gas tube 9 at the time of refrigerant replacement. It is characterized by that a separation recovery unit 6 having a separation recovery tank 61 is connected such that the separation recovery tank 61 is positioned in a path of an outdoor gas tube 29 connected to the gas tube 9, the air conditioning system 1 is operated in a cooling cycle, then, operation is carried out with the indoor expansion valve 51 closed, and containments stored in the separation recovery tank 61 is heated by a part of hot gas discharged from the compressor 11.