An air conditioner comprising: an outdoor heat exchanger; an outdoor fan for blowing air to the outdoor heat exchanger; and an outdoor fan motor that drives the outdoor fan; characterized by an outdoor fan inverter that drives the outdoor fan motor; and a control unit adapted to generate a rotation-speed command voltage for controlling the rotation number of the outdoor fan motor, and an actual rotation number of the outdoor fan is subjected to feedback to the control unit, a new rotation-speed command voltage for obtaining a target rotation number of the outdoor fan is output, and the rotation number of the outdoor fan is controlled to be constant, wherein the control unit is further adapted to start a defrost operation of the outdoor heat exchanger, based on the rotation-speed command voltage with which the rotation number of the outdoor fan is controlled to be constant; or wherein the control unit is further adapted to start the defrost operation of the outdoor heat exchanger, based on a duty ratio for generating the rotation-speed command voltage with which the rotation number of the outdoor fan is controlled to be constant, or an increase rate of a duty ratio; or wherein the control unit is further adapted to start the defrost operation of the outdoor heat exchanger, in a case where the actual rotation number of the outdoor fan motor that is subjected to feedback decreases by a value larger than or equal to a predetermined value with respect to a target rotation number, based on the rotation-speed command voltage with which the rotation number of the outdoor fan is controlled to be constant, and wherein the actual rotation number of the outdoor fan motor is detected and subjected to feedback to the control unit. an outdoor heat exchanger; an outdoor fan for blowing air to the outdoor heat exchanger; and an outdoor fan motor that drives the outdoor fan; characterized by an outdoor fan inverter that drives the outdoor fan motor; and a control unit adapted to generate a rotation-speed command voltage for controlling the rotation number of the outdoor fan motor, and an actual rotation number of the outdoor fan is subjected to feedback to the control unit, a new rotation-speed command voltage for obtaining a target rotation number of the outdoor fan is output, and the rotation number of the outdoor fan is controlled to be constant, wherein the control unit is further adapted to start a defrost operation of the outdoor heat exchanger, based on the rotation-speed command voltage with which the rotation number of the outdoor fan is controlled to be constant; or wherein the control unit is further adapted to start the defrost operation of the outdoor heat exchanger, based on a duty ratio for generating the rotation-speed command voltage with which the rotation number of the outdoor fan is controlled to be constant, or an increase rate of a duty ratio; or wherein the control unit is further adapted to start the defrost operation of the outdoor heat exchanger, in a case where the actual rotation number of the outdoor fan motor that is subjected to feedback decreases by a value larger than or equal to a predetermined value with respect to a target rotation number, based on the rotation-speed command voltage with which the rotation number of the outdoor fan is controlled to be constant, and wherein the actual rotation number of the outdoor fan motor is detected and subjected to feedback to the control unit.