A vehicle air-conditioning device comprising: - a refrigerant circuit including a compressor, an outdoor heat exchanger, an expansion device, and an indoor heat exchanger that are connected by refrigerant pipes to form a refrigeration cycle; - an indoor air-sending device that is adapted to supply air to the indoor heat exchanger; - an outdoor air-sending device that is adapted to supply air to the outdoor heat exchanger, wherein the refrigerant circuit is installed under the floor of a vehicle and uses carbon dioxide as the refrigerant, - a first chamber having a first air inlet through which air in the vehicle is introduced, and a first air outlet through which air blown out of the indoor air-sending device flows into the vehicle; and - a second chamber having a second air inlet through which air outside the vehicle is introduced, and a second air outlet through which air blown out of the outdoor air-sending device flows out of the vehicle, characterized by - a third chamber provided separately from the first chamber and the second chamber and having - a first opening communicating with the outside of the vehicle, a second opening communicating with the first chamber, an air supply duct connecting the first opening and the second opening, - a third opening communicating with the first chamber, a fourth opening communicating with the outside of the vehicle and an exhaust duct connecting the third opening and the fourth opening; - a supply ventilation air-sending device that sends air from the outside of the vehicle to the first chamber through the first opening, the air supply duct, and the second opening; - an exhaust ventilation air-sending device that discharges the air in the first chamber to the outside of the vehicle through the third opening, the exhaust duct, and the fourth opening; and - a ventilation damper that adjusts an amount of air supplied to the first chamber through the air supply duct and an amount of air discharged from the first chamber through the exhaust duct, wherein the first chamber has at least the indoor air-sending device and the indoor heat exchanger disposed therein, and the second chamber has at least the outdoor air-sending device and the outdoor heat exchanger disposed therein, and wherein the first chamber and the second chamber are provided separately, such that the air supplied to the outdoor heat exchanger and the air supplied to the indoor heat exchanger are not mixed. - a refrigerant circuit including a compressor, an outdoor heat exchanger, an expansion device, and an indoor heat exchanger that are connected by refrigerant pipes to form a refrigeration cycle; - an indoor air-sending device that is adapted to supply air to the indoor heat exchanger; - an outdoor air-sending device that is adapted to supply air to the outdoor heat exchanger, - a first chamber having a first air inlet through which air in the vehicle is introduced, and a first air outlet through which air blown out of the indoor air-sending device flows into the vehicle; and - a second chamber having a second air inlet through which air outside the vehicle is introduced, and a second air outlet through which air blown out of the outdoor air-sending device flows out of the vehicle, - a third chamber provided separately from the first chamber and the second chamber and having - a first opening communicating with the outside of the vehicle, a second opening communicating with the first chamber, an air supply duct connecting the first opening and the second opening, - a third opening communicating with the first chamber, a fourth opening communicating with the outside of the vehicle and an exhaust duct connecting the third opening and the fourth opening; - a supply ventilation air-sending device that sends air from the outside of the vehicle to the first chamber through the first opening, the air supply duct, and the second opening; - an exhaust ventilation air-sending device that discharges the air in the first chamber to the outside of the vehicle through the third opening, the exhaust duct, and the fourth opening; and - a ventilation damper that adjusts an amount of air supplied to the first chamber through the air supply duct and an amount of air discharged from the first chamber through the exhaust duct,