An air conditioner comprising: an outdoor unit comprising a compressor, an outdoor heat exchanger, a main expansion device, and a refrigerant pipe configured to connect the outdoor heat exchanger to the main expansion device; an indoor unit comprising an indoor heat exchanger; a connection pipe (171, 172) configured to connect the outdoor unit to the indoor unit, the air conditioner has refrigeration capacity of 16 kW to 28 kW, and a mixed refrigerant containing 50% or more of R32 is used as a refrigerant, characterised in that the refrigerant pipe is made of a ductile stainless steel material 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 ductile stainless steel material 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: 15.0 to 18.0%, 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, an average grain diameter of the austenite matrix structure is 30 μm to 60 μm, and the refrigerant pipe comprises a suction pipe that is configured to guide suction of the refrigerant into the compressor and wherein the suction pipe has an outer diameter of 19.05 mm and an inner diameter of 18.07 mm or less, wherein the refrigerant pipe further comprises a discharge pipe that guides discharge of the refrigerant compressed in the compressor and has an outer diameter of any one of 12.70 mm, 15.88 mm and 19.05 mm, wherein the discharge pipe has an inner diameter of 12.04 mm or less, when the discharge pipe has the outer diameter of 12.70 mm, wherein the discharge pipe has an inner diameter of 15.06 mm or less, when the discharge pipe has the outer diameter of 15.88 mm, and wherein the discharge pipe has an inner diameter of 18.07 mm or less, when the discharge pipe has the outer diameter of 19.05 mm. an outdoor unit comprising a compressor, an outdoor heat exchanger, a main expansion device, and a refrigerant pipe configured to connect the outdoor heat exchanger to the main expansion device; an indoor unit comprising an indoor heat exchanger; a connection pipe (171, 172) configured to connect the outdoor unit to the indoor unit, the air conditioner has refrigeration capacity of 16 kW to 28 kW, and a mixed refrigerant containing 50% or more of R32 is used as a refrigerant, characterised in that the refrigerant pipe is made of a ductile stainless steel material 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 ductile stainless steel material 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: 15.0 to 18.0%, 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, C: 0.03% or less, Si: more than 0 and less than 1.7%, Mn: 1.5 to 3.5%, Cr: 15.0 to 18.0%, 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, an average grain diameter of the austenite matrix structure is 30 μm to 60 μm, and the refrigerant pipe comprises a suction pipe that is configured to guide suction of the refrigerant into the compressor and wherein the suction pipe has an outer diameter of 19.05 mm and an inner diameter of 18.07 mm or less, wherein the refrigerant pipe further comprises a discharge pipe that guides discharge of the refrigerant compressed in the compressor and has an outer diameter of any one of 12.70 mm, 15.88 mm and 19.05 mm, wherein the discharge pipe has an inner diameter of 12.04 mm or less, when the discharge pipe has the outer diameter of 12.70 mm, wherein the discharge pipe has an inner diameter of 15.06 mm or less, when the discharge pipe has the outer diameter of 15.88 mm, and wherein the discharge pipe has an inner diameter of 18.07 mm or less, when the discharge pipe has the outer diameter of 19.05 mm.