An air conditioner includes an outdoor heat exchanger in which N heat exchange units are spaced apart from one another sequentially in a vertical direction N expansion valves respectively connected to the N heat exchange units; N hot gas pipes respectively connected to the N heat exchange units; N hot gas valves respectively installed in the N hot gas pipes; and a controller configured to control the N expansion valves and the N hot gas valves to defrost the N heat exchange units, wherein the controller is configured to defrost a bottom heat exchange unit first, then defrost a top heat exchange unit, and defrost the heat exchange units located under the top heat exchange unit sequentially from a second highest heat exchange unit to the bottom heat exchange unit after defrosting the top heat exchange unit.