An air conditioner is disclosed which includes a unit for setting a defrost mode, as required, when the heating operation mode is determined, a unit for enabling the refrigerant which is delivered from the compressor to flow through one or more indoor units calling for the heating operation mode, when the defrost mode is set, and to be returned back to the compressor through one or more indoor units calling for the cooling operation mode, and a unit for enabling a stream of the refrigerant which is delivered from the compressor to pass through the outdoor heat exchanger, when the defrost mode is set, and to enter the stream of the refrigerant flowing into one or more indoor units calling for the cooling operation mode.