An air conditioner comprising a compressor for compressing a refrigerant; a condenser for condensing the refrigerant compressed by the compressor; a phase separator for separating a gaseous refrigerant from the refrigerant passing through the condenser; an evaporator for evaporating the refrigerant passing through the phase separator; an inflow part provided in the phase separator to introduce the refrigerant into the phase separator; a gas separation part configured to discharge the gaseous refrigerant separated by the phase separator; an injection passage (235,250) configured to inject the refrigerant discharged from the phase separator into the compressor; and an internal heat-exchanger provided within the phase separator to heat-exchange a refrigerant therein with the refrigerant introduced through the inflow part; an outflow part provided in the phase separator to discharge the refrigerant introduced through the inflow part after the refrigerant introduced through the inflow part is heat-exchanged with the refrigerant within the internal heat-exchanger; a first inflow tube bypassing the refrigerant discharged from the outflow part to introduce the discharged refrigerant into the phase separator; a first outflow tube discharging the refrigerant introduced through the first inflow tube to the outside of the phase separator; wherein the injection passage (235, 250) comprises a low pressure injection passage for guiding injection of the refrigerant discharged through the first outflow tube into the compressor; and a high pressure injection passage for guiding injection of the refrigerant discharged through the gas separation part so that the refrigerant is injected into a high pressure part of the compressor, wherein: the compressor comprises a first compression part for compressing the refrigerant passing through the evaporator, a second compression part for compressing the refrigerant compressed by the first compression part, and a third compression part for compressing the refrigerant compressed by the second compression part, characterized in that: the refrigerant discharged from the gas separation part flows into between the second compression part and the third compression part and the refrigerant discharged through the first outflow tube flows into between the first compression part and the second compression part, the condenser or the evaporator is an indoor heat-exchanger, and the air conditioner further comprises: a first connection passage connecting the outflow part to the indoor heat-exchanger; a first check valve provided in the first connection passage to guide the refrigerant in one direction; a second connection passage branched from one point of a passage defined between the first check valve and the indoor heat-exchanger to communicate with the inflow part; and a second check valve provided in the second connection passage to guide a refrigerant flow in a direction of the inflow part. the compressor comprises a first compression part for compressing the refrigerant passing through the evaporator, a second compression part for compressing the refrigerant compressed by the first compression part, and a third compression part for compressing the refrigerant compressed by the second compression part, characterized in that: the refrigerant discharged from the gas separation part flows into between the second compression part and the third compression part and the refrigerant discharged through the first outflow tube flows into between the first compression part and the second compression part, the condenser or the evaporator is an indoor heat-exchanger, and the air conditioner further comprises: a first connection passage connecting the outflow part to the indoor heat-exchanger; a first check valve provided in the first connection passage to guide the refrigerant in one direction; a second connection passage branched from one point of a passage defined between the first check valve and the indoor heat-exchanger to communicate with the inflow part; and a second check valve provided in the second connection passage to guide a refrigerant flow in a direction of the inflow part. the refrigerant discharged from the gas separation part flows into between the second compression part and the third compression part and the refrigerant discharged through the first outflow tube flows into between the first compression part and the second compression part, the condenser or the evaporator is an indoor heat-exchanger, and the air conditioner further comprises: a first connection passage connecting the outflow part to the indoor heat-exchanger; a first check valve provided in the first connection passage to guide the refrigerant in one direction; a second connection passage branched from one point of a passage defined between the first check valve and the indoor heat-exchanger to communicate with the inflow part; and a second check valve provided in the second connection passage to guide a refrigerant flow in a direction of the inflow part. a first connection passage connecting the outflow part to the indoor heat-exchanger; a first check valve provided in the first connection passage to guide the refrigerant in one direction; a second connection passage branched from one point of a passage defined between the first check valve and the indoor heat-exchanger to communicate with the inflow part; and a second check valve provided in the second connection passage to guide a refrigerant flow in a direction of the inflow part.