An air-conditioner, comprising: a return-air inlet and a supply-air outlet each communicating with a predetermined space; a first main air channel configured to allow air to flow therein towards the supply-air outlet; a first heat exchanger disposed in the first main air channel to cause a heat-exchange between refrigerant flowing therein and air passing therethrough; an outside-air inlet and an exhaust-air outlet communicating with an outside of the predetermined space; a second main air channel configured to allow air to flow therein from the outside-air inlet towards the exhaust-air outlet; a second heat exchanger disposed in the second main air channel to cause a heat-exchange between refrigerant flowing therein and air passing therethrough; and an exhaust ventilation channel configured to allow air to flow therein from the return-air inlet towards the exhaust-air outlet, wherein: the exhaust ventilation channel is configured to allow ventilation exhaust air to pass through the second heat exchanger, the ventilation exhaust air being air that has been taken in through the return-air inlet and to be discharged through the exhaust-air outlet, the air-conditioner further comprising an airflow regulator configured to regulate a flow of at least second passing air so as to prevent the second passing air from passing through the second heat exchanger, the second passing air being air that has been taken in through the outside-air inlet and to be discharged through the exhaust-air outlet; characterized in that each of the first and second heat exchangers is configured to flow carbon dioxide refrigerant therein; and the airflow regulator is further configured to regulate the flows of the second passing air and the ventilation exhaust air such that the temperature of an air which goes through the second heat exchanger is under the supercritical temperature of the carbon dioxide refrigerant flowing in the second heat exchanger. a return-air inlet and a supply-air outlet each communicating with a predetermined space; a first main air channel configured to allow air to flow therein towards the supply-air outlet; a first heat exchanger disposed in the first main air channel to cause a heat-exchange between refrigerant flowing therein and air passing therethrough; an outside-air inlet and an exhaust-air outlet communicating with an outside of the predetermined space; a second main air channel configured to allow air to flow therein from the outside-air inlet towards the exhaust-air outlet; a second heat exchanger disposed in the second main air channel to cause a heat-exchange between refrigerant flowing therein and air passing therethrough; and an exhaust ventilation channel configured to allow air to flow therein from the return-air inlet towards the exhaust-air outlet, wherein: the exhaust ventilation channel is configured to allow ventilation exhaust air to pass through the second heat exchanger, the ventilation exhaust air being air that has been taken in through the return-air inlet and to be discharged through the exhaust-air outlet, the air-conditioner further comprising an airflow regulator configured to regulate a flow of at least second passing air so as to prevent the second passing air from passing through the second heat exchanger, the second passing air being air that has been taken in through the outside-air inlet and to be discharged through the exhaust-air outlet; characterized in that the exhaust ventilation channel is configured to allow ventilation exhaust air to pass through the second heat exchanger, the ventilation exhaust air being air that has been taken in through the return-air inlet and to be discharged through the exhaust-air outlet, the air-conditioner further comprising an airflow regulator configured to regulate a flow of at least second passing air so as to prevent the second passing air from passing through the second heat exchanger, the second passing air being air that has been taken in through the outside-air inlet and to be discharged through the exhaust-air outlet; characterized in that the ventilation exhaust air being air that has been taken in through the return-air inlet and to be discharged through the exhaust-air outlet, the air-conditioner further comprising an airflow regulator configured to regulate a flow of at least second passing air so as to prevent the second passing air from passing through the second heat exchanger, the second passing air being air that has been taken in through the outside-air inlet and to be discharged through the exhaust-air outlet; characterized in that each of the first and second heat exchangers is configured to flow carbon dioxide refrigerant therein; and the airflow regulator is further configured to regulate the flows of the second passing air and the ventilation exhaust air such that the temperature of an air which goes through the second heat exchanger is under the supercritical temperature of the carbon dioxide refrigerant flowing in the second heat exchanger.