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.