An air conditioner, comprising: an outdoor unit including a compressor, a four-way valve, an outdoor heat exchanger, a heat exchanger outlet temperature detector for detecting a heat exchanger outlet temperature providing the temperature of a refrigerant flowing out from the outdoor heat exchanger when the outdoor heat exchanger functions as an evaporator, and an outdoor air temperature detector for detecting an outdoor air temperature; a plurality of indoor units (5a, 5b, 5c) respectively including indoor heat exchangers (51a, 51b, 51c); a refrigerant circuit constituted of the compressor, the four-way valve, the outdoor heat exchanger and the indoor heat exchangers (51a, 51b, 51c) connected to each other by refrigerant pipes; and, a control unit for controlling the compressor and the four-way valve, wherein the control unit, while the plurality of indoor units (5a, 5b, 5c) are performing a heating operation with the refrigerant circuit set in a heating cycle, inputs periodically heat exchanger outlet temperatures detected by the heat exchanger outlet temperature detector, outdoor air temperatures detected by the outdoor air temperature detector and the number of the indoor units (5a, 5b, 5c) performing the heating operation, and stores the thus-input data in time series, wherein the control unit, when any one of first and second frost forming conditions serving as the condition for frost formation in the outdoor heat exchanger is satisfied, switches the four-way valve to set the refrigerant circuit into a cooling cycle to thereby execute a defrosting operation for defrosting the outdoor heat exchanger, wherein the first frost forming condition is satisfied when the heat exchange outlet temperature is lower than a first threshold temperature, when the duration of the heating operation is longer than a specific duration, and when a temperature difference between the outdoor air temperature and the heat exchanger outlet temperature is larger than a specific temperature difference, and wherein the second frost forming condition is satisfied when, of the heat exchanger outlet temperatures stored in time series, the latest heat exchanger outlet temperature is lower than the a second threshold temperature higher than the first threshold temperature, when the drop rate of the heat exchanger outlet temperature calculated using two continuing heat exchanger outlet temperatures of the heat exchanger outlet temperatures stored in time series is larger than a specific drop rate, and when the number of indoor units (5a, 5b, 5c) in operation has increased during calculation of the drop rate. an outdoor unit including a compressor, a four-way valve, an outdoor heat exchanger, a heat exchanger outlet temperature detector for detecting a heat exchanger outlet temperature providing the temperature of a refrigerant flowing out from the outdoor heat exchanger when the outdoor heat exchanger functions as an evaporator, and an outdoor air temperature detector for detecting an outdoor air temperature; a plurality of indoor units (5a, 5b, 5c) respectively including indoor heat exchangers (51a, 51b, 51c); a refrigerant circuit constituted of the compressor, the four-way valve, the outdoor heat exchanger and the indoor heat exchangers (51a, 51b, 51c) connected to each other by refrigerant pipes; and, a control unit for controlling the compressor and the four-way valve, wherein the control unit, while the plurality of indoor units (5a, 5b, 5c) are performing a heating operation with the refrigerant circuit set in a heating cycle, inputs periodically heat exchanger outlet temperatures detected by the heat exchanger outlet temperature detector, outdoor air temperatures detected by the outdoor air temperature detector and the number of the indoor units (5a, 5b, 5c) performing the heating operation, and stores the thus-input data in time series, wherein the control unit, when any one of first and second frost forming conditions serving as the condition for frost formation in the outdoor heat exchanger is satisfied, switches the four-way valve to set the refrigerant circuit into a cooling cycle to thereby execute a defrosting operation for defrosting the outdoor heat exchanger, wherein the first frost forming condition is satisfied when the heat exchange outlet temperature is lower than a first threshold temperature, when the duration of the heating operation is longer than a specific duration, and when a temperature difference between the outdoor air temperature and the heat exchanger outlet temperature is larger than a specific temperature difference, and wherein the second frost forming condition is satisfied when, of the heat exchanger outlet temperatures stored in time series, the latest heat exchanger outlet temperature is lower than the a second threshold temperature higher than the first threshold temperature, when the drop rate of the heat exchanger outlet temperature calculated using two continuing heat exchanger outlet temperatures of the heat exchanger outlet temperatures stored in time series is larger than a specific drop rate, and when the number of indoor units (5a, 5b, 5c) in operation has increased during calculation of the drop rate.