To provide an air conditioner shortening a defrost operation time regardless of an amount of frost formation of an outdoor heat exchanger, and securely eliminating frost formation.

The air conditioner is provided with an outdoor unit equipped with: the outdoor heat exchanger for heat exchanging with the outdoor air; a compressor for compressing coolant; an outdoor heat exchanging temperature sensor which is a means for detecting temperature of the heat exchanger; and a control device having a normal defrost operation mode (S7) for supplying the coolant compressed by the compressor and giving a heat amount to the outdoor heat exchanger to eliminate the adhering frost. The control device is characterized by choosing (S3) either the normal defrost operation mode (S7) or a reinforced defrost operation mode (S8) in which a frequency of the compressor is increased to be higher than a compressor frequency in the defrost operation mode based on temperature information of the outdoor heat exchanging temperature sensor after passing (S2) a predetermined time (S) from starting of the defrost operation (S1).