An outdoor unit of an air conditioner comprising: a refrigerant circuit configured to circulate a refrigerant between a compressor, an indoor heat exchanger (51a to 51c), and an outdoor heat exchanger; a flow path switch unit included in the refrigerant circuit and configured to switch a flow direction of the refrigerant discharged from the compressor; an outdoor fan; an outdoor air temperature detector configured to detect an outdoor air temperature; and a controller configured to control the outdoor fan and the refrigerant circuit, wherein the controller performs: a fan defrost operation to circulate the refrigerant through the compressor, the outdoor heat exchanger, and the indoor heat exchanger (51a to 51c) in this order same as in the case of a cooling operation and rotate the outdoor fan when the outdoor air temperature detected by the outdoor air temperature detector is within a predetermined temperature range, characterized in that the controller performs: a fan defrost operation over a period of a first fan defrost operation time when the outdoor air temperature detected by the outdoor air temperature detector is lower than a first predetermined temperature, and a fan defrost operation over a period of a second fan defrost operation time that is longer than the first fan defrost operation time when the outdoor air temperature detected by the outdoor air temperature detector is equal to or higher than a second predetermined temperature that is higher than the first predetermined temperature. a refrigerant circuit configured to circulate a refrigerant between a compressor, an indoor heat exchanger (51a to 51c), and an outdoor heat exchanger; a flow path switch unit included in the refrigerant circuit and configured to switch a flow direction of the refrigerant discharged from the compressor; an outdoor fan; an outdoor air temperature detector configured to detect an outdoor air temperature; and a controller configured to control the outdoor fan and the refrigerant circuit, wherein the controller performs: a fan defrost operation to circulate the refrigerant through the compressor, the outdoor heat exchanger, and the indoor heat exchanger (51a to 51c) in this order same as in the case of a cooling operation and rotate the outdoor fan when the outdoor air temperature detected by the outdoor air temperature detector is within a predetermined temperature range, characterized in that the controller performs: a fan defrost operation over a period of a first fan defrost operation time when the outdoor air temperature detected by the outdoor air temperature detector is lower than a first predetermined temperature, and a fan defrost operation over a period of a second fan defrost operation time that is longer than the first fan defrost operation time when the outdoor air temperature detected by the outdoor air temperature detector is equal to or higher than a second predetermined temperature that is higher than the first predetermined temperature. a fan defrost operation over a period of a first fan defrost operation time when the outdoor air temperature detected by the outdoor air temperature detector is lower than a first predetermined temperature, and a fan defrost operation over a period of a second fan defrost operation time that is longer than the first fan defrost operation time when the outdoor air temperature detected by the outdoor air temperature detector is equal to or higher than a second predetermined temperature that is higher than the first predetermined temperature.