An air conditioner comprising: a compressor which is configured to compress a refrigerant; a condenser (4, 5) which is configured to condense the refrigerant discharged from the compressor; an expansion valve (Va, Vb) which is configured to expand the refrigerant passed through the condenser (4, 5); a gas-liquid separation pipe (11a, 11b) through which the refrigerant passed through the expansion valve flows during operation of the air conditioner; a gas-liquid separator, through which the refrigerant passed through the gas-liquid separation pipe (11a, 12b) is introduced, that separates and discharges the refrigerant introduced into the gas-liquid separator into a vapor refrigerant and a liquid refrigerant during operation of the air conditioner; and an evaporator (5, 4) which is configured to evaporate the liquid refrigerant discharged from the gas-liquid separator, wherein the gas-liquid separator is formed in a shape of cylinder extending in an up and downward direction, wherein a vapor refrigerant pipe is connected between the gas-liquid separator and the compressor and an injection valve (Vi) is installed in the vapor refrigerant pipe to open and close a flow path for refrigerant; wherein, during operation of the air conditioner, the vapor refrigerant discharged from the gas-liquid separator is provided to the compressor via the vapor refrigerant pipe and the liquid refrigerant discharged from the gas-liquid separator is provided to the evaporator (5, 4), and characterized in that the gas-liquid separation pipe (11a, 12a) comprises: a first part (111, 121) which extends in a horizontal direction, and one end thereof is connected to a refrigerant inflow pipe in which the expansion valve (Va) is installed; and a second part (112, 122) which extends in a vertical direction from the first part (111, 121) and one end thereof is coupled to the first part (111, 121), wherein one of the other end of the first part (111, 121), and the other end of the second part (112, 122) is connected to an upper part of the gas-liquid separator, and the other one of the other end of first part (111, 121) and the other end of the second part (112, 122) is connected to a lower part of the gas-liquid separator. a compressor which is configured to compress a refrigerant; a condenser (4, 5) which is configured to condense the refrigerant discharged from the compressor; an expansion valve (Va, Vb) which is configured to expand the refrigerant passed through the condenser (4, 5); a gas-liquid separation pipe (11a, 11b) through which the refrigerant passed through the expansion valve flows during operation of the air conditioner; a gas-liquid separator, through which the refrigerant passed through the gas-liquid separation pipe (11a, 12b) is introduced, that separates and discharges the refrigerant introduced into the gas-liquid separator into a vapor refrigerant and a liquid refrigerant during operation of the air conditioner; and an evaporator (5, 4) which is configured to evaporate the liquid refrigerant discharged from the gas-liquid separator, wherein the gas-liquid separator is formed in a shape of cylinder extending in an up and downward direction, wherein a vapor refrigerant pipe is connected between the gas-liquid separator and the compressor and an injection valve (Vi) is installed in the vapor refrigerant pipe to open and close a flow path for refrigerant; wherein, during operation of the air conditioner, the vapor refrigerant discharged from the gas-liquid separator is provided to the compressor via the vapor refrigerant pipe and the liquid refrigerant discharged from the gas-liquid separator is provided to the evaporator (5, 4), and characterized in that the gas-liquid separation pipe (11a, 12a) comprises: a first part (111, 121) which extends in a horizontal direction, and one end thereof is connected to a refrigerant inflow pipe in which the expansion valve (Va) is installed; and a second part (112, 122) which extends in a vertical direction from the first part (111, 121) and one end thereof is coupled to the first part (111, 121), wherein one of the other end of the first part (111, 121), and the other end of the second part (112, 122) is connected to an upper part of the gas-liquid separator, and the other one of the other end of first part (111, 121) and the other end of the second part (112, 122) is connected to a lower part of the gas-liquid separator. a first part (111, 121) which extends in a horizontal direction, and one end thereof is connected to a refrigerant inflow pipe in which the expansion valve (Va) is installed; and a second part (112, 122) which extends in a vertical direction from the first part (111, 121) and one end thereof is coupled to the first part (111, 121), wherein one of the other end of the first part (111, 121), and the other end of the second part (112, 122) is connected to an upper part of the gas-liquid separator, and the other one of the other end of first part (111, 121) and the other end of the second part (112, 122) is connected to a lower part of the gas-liquid separator.