Disclosed is an air-conditioning apparatus using a heat pipe. The state of the outside air is compared with the state of set supply air. The heat exchange and mixed supply of outside air and ventilation air are effectively performed by changing the passage of the ventilation air and the outside air through the selective opening/shutting of dampers, cooling the outside air through latent heat by spraying mist, and by controlling supply air in a set state through the cooling and humidification of the supply air. Accordingly, energy efficiency can be improved and the energy necessary for the air-conditioning apparatus can be reduced using the evaporation latent heat of water. Furthermore, operation costs can be reduced and financial gains can be obtained because an efficient operation can be performed in response to the state of a measured outside air.