An air conditioner comprising: a refrigerant circuit including: a compressor; an outdoor heat exchanger; an expansion valve; and an indoor heat exchanger all of which are connected in a stated order; the refrigerant circuit further including means for reversing refrigerant circulation; a cycle controller causing, by control of the means for reversing refrigerant circulation, either (i) the outdoor heat exchanger to function as an evaporator and the indoor heat exchanger to function as a condenser to create a heating cycle in the refrigerant circuit or (ii) the outdoor heat exchanger to function as the condenser and the indoor heat exchanger to function as the evaporator when a reverse cycle executing condition is met, to create a reverse cycle in the refrigerant circuit, so that the refrigerant circulates in reverse of the heating cycle; and a rotation speed controller adjusting a rotation speed of the compressor in the reverse cycle, depending on an index correlated with an amount of frost on the outdoor heat exchanger at a start of the reverse cycle, the rotation speed controller decreasing the rotation speed of the compressor in the reverse cycle as the index at the start of the reverse cycle indicates that the amount of the frost on the outdoor heat exchanger is smaller, the rotation speed controller comprising: means for obtaining the outdoor temperature (Ta); and means for obtaining the temperature (Tr) of the outside surface of the outdoor heat exchanger; the air conditioning system being characterized in that the rotation speed controller determines that the amount of frost on the outside surface of the outdoor heat exchanger is smaller as the temperature (Tr) of the outside surface of the outdoor heat exchanger is higher with respect to the outdoor temperature (Ta), or, the rotation speed controller determines that the amount of frost is greater as the temperature (Tr) of the outdoor heat exchanger is lower with respect to the outdoor temperature (Ta). a refrigerant circuit including: a compressor; an outdoor heat exchanger; an expansion valve; and an indoor heat exchanger all of which are connected in a stated order; the refrigerant circuit further including means for reversing refrigerant circulation; a cycle controller causing, by control of the means for reversing refrigerant circulation, either (i) the outdoor heat exchanger to function as an evaporator and the indoor heat exchanger to function as a condenser to create a heating cycle in the refrigerant circuit or (ii) the outdoor heat exchanger to function as the condenser and the indoor heat exchanger to function as the evaporator when a reverse cycle executing condition is met, to create a reverse cycle in the refrigerant circuit, so that the refrigerant circulates in reverse of the heating cycle; and a rotation speed controller adjusting a rotation speed of the compressor in the reverse cycle, depending on an index correlated with an amount of frost on the outdoor heat exchanger at a start of the reverse cycle, the rotation speed controller decreasing the rotation speed of the compressor in the reverse cycle as the index at the start of the reverse cycle indicates that the amount of the frost on the outdoor heat exchanger is smaller, the rotation speed controller comprising: means for obtaining the outdoor temperature (Ta); and means for obtaining the temperature (Tr) of the outside surface of the outdoor heat exchanger; means for obtaining the outdoor temperature (Ta); and means for obtaining the temperature (Tr) of the outside surface of the outdoor heat exchanger; the air conditioning system being characterized in that the rotation speed controller determines that the amount of frost on the outside surface of the outdoor heat exchanger is smaller as the temperature (Tr) of the outside surface of the outdoor heat exchanger is higher with respect to the outdoor temperature (Ta), or, the rotation speed controller determines that the amount of frost is greater as the temperature (Tr) of the outdoor heat exchanger is lower with respect to the outdoor temperature (Ta).