Disclosed is an air conditioner for attempting to improve on the corrosion of the outdoor unit. The air conditioner of the present invention for performing a cooling operation and a heating operation by switching a four side valve, that includes an outdoor side heat exchanger operating as a condenser during the cooling operation and an evaporator during the heating operation, and having fins and heat transfer tubes, wherein the outdoor side heat exchanger is placed on a baseboard which configures a lower portion of a chassis of the outdoor unit, comprises the fins and the heat transfer tubes of the outdoor side heat exchanger that are constructed with aluminum or aluminum alloy, and the baseboard that is constructed with Zn—Al plated steel board or Zn—Al—Mg plated steel board.