The invention provides an air conditioner for a vehicle. The air conditioner for a vehicle (10) includes: a compressor (21) that compresses a heat exchange medium; an internal heat exchanger (14) that performs heat exchange between the heat exchange medium expelled from the compressor (21) and air for air conditioning introduced inside a vehicle compartment; and an external heat exchanger (24) that performs heat exchange between the heat exchange medium expelled from the internal heat exchanger (14) and external air, wherein the air conditioner for a vehicle (10) calculates (S10) a temperature difference (Tn) between an external air temperature (Tam) and a discharge port temperature (Tout) of the external heat exchanger (24), calculates (S 14) an average value (Ax, Ay) of the temperature difference (Tn), and performs (S 18) a defrosting operation which melts frost adhered to the external heat exchanger (24) when an amount of change (Ay-Ax) of the average value (Ax, Ay) is equal to or greater than a first predetermined value (dA1).