An air conditioner comprising: an indoor device installed at indoor; an outdoor device connected to the indoor device using a refrigerant pipe; a ventilation unit for exchanging heat of outdoor air and indoor air while ventilating indoor air and outdoor air; and a guidance duct for communicating the ventilation unit and an air inhalation portion of the outdoor device in order to guide indoor air exhausted from the ventilation unit to an air inhalation portion formed in the outdoor device, wherein the ventilation unit comprises a ventilation heat exchanger for exchanging heat of indoor air and outdoor air; ventilation ventilators, for allowing indoor air to pass through the ventilation heat exchanger, sending the indoor air to the outdoor device, allowing outdoor air to pass through the ventilation heat exchanger, and sending the outdoor air to indoor; and a ventilation case having the ventilation heat exchanger and a ventilation fan therein, wherein a ventilation case has an outdoor air inhalation duct connection part to which an outdoor air inhalation air duct is connected, an outdoor air exhaust duct connection part to which an outdoor air exhaust duct is connected, an indoor air inhalation duct connection part to which an indoor air inhalation duct is connected, and a guidance duct connection part to which the guidance duct is connected, wherein the outdoor device comprises a base, left and right side panels (36, 38) installed at the upper side of the base, a top panel installed at the upper side of the left and right side panels, and an outdoor heat exchanger for exchanging heat of outdoor air and a refrigerant, characterised in that the left and right side panels, have the air inhalation portion at a position opposite to the outdoor heat exchanger, wherein at least one of the left and right side panels, has a plurality of openings forming the air inhalation portion, wherein the guidance duct has a fastening portion fastened to the outdoor device in a variable fastening height. an indoor device installed at indoor; an outdoor device connected to the indoor device using a refrigerant pipe; a ventilation unit for exchanging heat of outdoor air and indoor air while ventilating indoor air and outdoor air; and a guidance duct for communicating the ventilation unit and an air inhalation portion of the outdoor device in order to guide indoor air exhausted from the ventilation unit to an air inhalation portion formed in the outdoor device, wherein the ventilation unit comprises a ventilation heat exchanger for exchanging heat of indoor air and outdoor air; ventilation ventilators, for allowing indoor air to pass through the ventilation heat exchanger, sending the indoor air to the outdoor device, allowing outdoor air to pass through the ventilation heat exchanger, and sending the outdoor air to indoor; and a ventilation case having the ventilation heat exchanger and a ventilation fan therein, wherein a ventilation case has an outdoor air inhalation duct connection part to which an outdoor air inhalation air duct is connected, an outdoor air exhaust duct connection part to which an outdoor air exhaust duct is connected, an indoor air inhalation duct connection part to which an indoor air inhalation duct is connected, and a guidance duct connection part to which the guidance duct is connected, wherein the outdoor device comprises a base, left and right side panels (36, 38) installed at the upper side of the base, a top panel installed at the upper side of the left and right side panels, and an outdoor heat exchanger for exchanging heat of outdoor air and a refrigerant, characterised in that the left and right side panels, have the air inhalation portion at a position opposite to the outdoor heat exchanger, wherein at least one of the left and right side panels, has a plurality of openings forming the air inhalation portion, wherein the guidance duct has a fastening portion fastened to the outdoor device in a variable fastening height.