Disclosed herein is an air conditioner having a single oil separator mounted on a confluent discharging channel, to which are connected discharging channels provided at a plurality of compressors, and an electronic expansion valve, of which the opening level is controllable, mounted on an oil collection pipe connected to the oil separator. As a result, the number of the oil separators is decreased. Consequently, the present invention has the effect of simplifying the structure of the air conditioner and reducing the manufacturing costs of the air conditioner. Furthermore, refrigerant is bypassed by controlling the opening level of the electronic expansion valve to control the flow rate of the refrigerant. Consequently, the present invention has the effect of eliminating an additional bypassing device, and therefore, simplifying the structure of the air conditioner and the controlling method of the air conditioner.