A control method of a multi air conditioner comprising an outdoor unit having an inverter type compressor, an outdoor heat exchanger, and a power detector, and a plurality of indoor units each having an indoor heat exchanger (210, 310), said power detector configured to detect power on/off signals of a second indoor unit of said plurality of indoor units, the method comprising: judging whether, among the plurality of indoor units (200, 300), a first indoor unit, which communicates with the outdoor unit, and said second indoor unit, which does not communicate with the outdoor unit, are operated respectively; controlling driving of the compressor at a predetermined frequency and controlling an opening rate of a second flow-rate adjustable valve to adjust a flow rate of refrigerant to be fed to the second indoor unit based on a predetermined frequency of the compressor, predetermined indoor information of the second indoor unit, and outdoor information detected by the outdoor unit, if solo operation of the second indoor unit is judged; and controlling a frequency of the compressor based on capacities of the first and second indoor units (200, 300) and indoor information transmitted from the first indoor unit, and controlling opening rates of first and second flow-rate adjustable valves (125, 130) respectively to adjust flow rates of refrigerant to be fed to the first and second indoor units (200, 300) based on indoor information of the first indoor unit, the frequency of the compressor and outdoor information detected by the outdoor unit and predetermined indoor information of the second indoor unit, if simultaneous operation of the first and second indoor units (125, 130) is judged, wherein the judging whether or not the second indoor unit is operated, includes judging whether or not an operating command of the second indoor unit is input by detecting a power on signal applied to the second indoor unit via the power detector. judging whether, among the plurality of indoor units (200, 300), a first indoor unit, which communicates with the outdoor unit, and said second indoor unit, which does not communicate with the outdoor unit, are operated respectively; controlling driving of the compressor at a predetermined frequency and controlling an opening rate of a second flow-rate adjustable valve to adjust a flow rate of refrigerant to be fed to the second indoor unit based on a predetermined frequency of the compressor, predetermined indoor information of the second indoor unit, and outdoor information detected by the outdoor unit, if solo operation of the second indoor unit is judged; and controlling a frequency of the compressor based on capacities of the first and second indoor units (200, 300) and indoor information transmitted from the first indoor unit, and controlling opening rates of first and second flow-rate adjustable valves (125, 130) respectively to adjust flow rates of refrigerant to be fed to the first and second indoor units (200, 300) based on indoor information of the first indoor unit, the frequency of the compressor and outdoor information detected by the outdoor unit and predetermined indoor information of the second indoor unit, if simultaneous operation of the first and second indoor units (125, 130) is judged, wherein the judging whether or not the second indoor unit is operated, includes judging whether or not an operating command of the second indoor unit is input by detecting a power on signal applied to the second indoor unit via the power detector.