An air conditioner comprising: an outdoor unit comprising a compressor , an outdoor heat exchanger , and a main expansion device, wherein a refrigerant is circulated by a refrigerant pipe configured to connect the compressor , the outdoor heat exchanger , and the main expansion device, an indoor unit comprising an indoor heat exchanger; and a connection pipe configured to connect the outdoor unit and the indoor unit, wherein the refrigerant pipe comprises a suction pipe that is configured to guide suction of the refrigerant to the compressor , and a discharge pipe that is configured to guide discharge of the refrigerant from the compressor , wherein the air conditioner has a cooling capability between 23 kW and 35 kW, wherein the air conditioner comprises a refrigerant and a mixed refrigerant containing R32 of 50% or more is used as the refrigerant, characterised in that the refrigerant pipe is made of a ductile stainless steel having an austenite matrix structure of 99% or more and a delta ferrite matrix structure of 1% or less, on the basis of a grain area, wherein the stainless steel comprises, percent by weight: C: 0.03% or less; Si: more than 0% and less than 1.7%; Mn: 1.5% to 3.5%; Cr: 15% to 18%; Ni: 7.0% to 9.0%; Cu: 1.0% to 4.0%; Mo: 0.03% or less; P: 0.04% or less; S: 0.04% or less; and N: 0.03% or less; and an average grain diameter of the austenite matrix structure is 30 μm to 60 μm; wherein the suction pipe has an outer diameter of 22.20 mm and an inner diameter of 21.06 mm or less; and wherein the discharge pipe has an outer diameter of 15.88 mm and a inner diameter of 15.06 mm or less. an outdoor unit comprising a compressor , an outdoor heat exchanger , and a main expansion device, wherein a refrigerant is circulated by a refrigerant pipe configured to connect the compressor , the outdoor heat exchanger , and the main expansion device, an indoor unit comprising an indoor heat exchanger; and a connection pipe configured to connect the outdoor unit and the indoor unit, wherein the refrigerant pipe comprises a suction pipe that is configured to guide suction of the refrigerant to the compressor , and a discharge pipe that is configured to guide discharge of the refrigerant from the compressor , wherein the air conditioner has a cooling capability between 23 kW and 35 kW, wherein the air conditioner comprises a refrigerant and a mixed refrigerant containing R32 of 50% or more is used as the refrigerant, characterised in that the refrigerant pipe is made of a ductile stainless steel having an austenite matrix structure of 99% or more and a delta ferrite matrix structure of 1% or less, on the basis of a grain area, wherein the stainless steel comprises, percent by weight: C: 0.03% or less; Si: more than 0% and less than 1.7%; Mn: 1.5% to 3.5%; Cr: 15% to 18%; Ni: 7.0% to 9.0%; Cu: 1.0% to 4.0%; Mo: 0.03% or less; P: 0.04% or less; S: 0.04% or less; and N: 0.03% or less; and an average grain diameter of the austenite matrix structure is 30 μm to 60 μm; wherein the suction pipe has an outer diameter of 22.20 mm and an inner diameter of 21.06 mm or less; and wherein the discharge pipe has an outer diameter of 15.88 mm and a inner diameter of 15.06 mm or less. <EOS>