An air conditioner comprising: a casing ; and a blower and a refrigerant circuit disposed in the casing , wherein the blower is configured to blow air, the refrigerant circuit has a compressor , a condenser , a decompressor , and an evaporator and is configured to circulate refrigerant in order of the compressor , the condenser , the decompressor , and the evaporator , the condenser has a first heat transfer tube through which the refrigerant flows and which has a first outside diameter, the evaporator has a second heat transfer tube through which the refrigerant flows and which has a second outside diameter, the evaporator is disposed windward of the condenser , and the first outside diameter of the first heat transfer tube of the condenser is smaller than the second outside diameter of the second heat transfer tube of the evaporator ; wherein the second heat transfer tube of the evaporator is a circular tube; and characterised in that the first heat transfer tube of the condenser is a flat tube; the first heat transfer tube has a cross-section that extends in a direction in which the evaporator and the condenser are aligned; the first heat transfer tube of the condenser is disposed in a region which is less occupied by the second heat transfer tube of the evaporator in the direction in which the evaporator and the condenser are aligned; the first heat transfer tube of the condenser is configured to enable heat transfer at a leading edge of the first heat transfer tube of the condenser ; a material for the condenser has a pitting potential higher than a pitting potential of a material for the evaporator ; and the pitting potentials are set such that a fin of the evaporator ≤ a fin of the condenser < the second heat transfer tube of the evaporator < the first heat transfer tube of the condenser . a casing ; and a blower and a refrigerant circuit disposed in the casing , wherein the blower is configured to blow air, the refrigerant circuit has a compressor , a condenser , a decompressor , and an evaporator and is configured to circulate refrigerant in order of the compressor , the condenser , the decompressor , and the evaporator , the condenser has a first heat transfer tube through which the refrigerant flows and which has a first outside diameter, the evaporator has a second heat transfer tube through which the refrigerant flows and which has a second outside diameter, the evaporator is disposed windward of the condenser , and the first outside diameter of the first heat transfer tube of the condenser is smaller than the second outside diameter of the second heat transfer tube of the evaporator ; wherein the second heat transfer tube of the evaporator is a circular tube; and characterised in that the first heat transfer tube of the condenser is a flat tube; the first heat transfer tube has a cross-section that extends in a direction in which the evaporator and the condenser are aligned; the first heat transfer tube of the condenser is disposed in a region which is less occupied by the second heat transfer tube of the evaporator in the direction in which the evaporator and the condenser are aligned; the first heat transfer tube of the condenser is configured to enable heat transfer at a leading edge of the first heat transfer tube of the condenser ; a material for the condenser has a pitting potential higher than a pitting potential of a material for the evaporator ; and the pitting potentials are set such that a fin of the evaporator ≤ a fin of the condenser < the second heat transfer tube of the evaporator < the first heat transfer tube of the condenser . <EOS>