An air conditioning apparatus installed on a vehicle, the air conditioning apparatus comprising: a casing installed on the vehicle and defining an indoor chamber, the indoor chamber communicating with a vehicular compartment of the vehicle; an indoor heat exchanger arranged in the indoor chamber and having an interior for flow therethrough of a refrigerant containing carbon dioxide; a cooperation device (110a, 110b, 120a, 120b, 140a, 140b, 150a, 150b) included in, together with the indoor heat exchanger, a refrigeration cycle device for circulation therethrough of the refrigerant; the cooperation device (110a, 110b, 120a, 120b, 140a, 140b, 150a, 150b) comprising a compressor (110a;110b), an outdoor heat exchanger (120a;120b), an expander, an accumulator (140a; 140b) and a refrigerant piping (150a; 150b); an indoor fan arranged in the indoor chamber and configured to form airflow, the airflow being formed by air flowing from the vehicular compartment into the indoor chamber, passing through the indoor heat exchanger, and flowing out to return to the vehicular compartment; and a first gas sensor to detect a concentration of carbon dioxide gas in a path of the airflow at a position located downstream of the indoor heat exchanger, wherein the first gas sensor includes: a sensing portion configured to react in accordance with the concentration of the carbon dioxide gas, a main body fixed to the casing; a protrusion portion that protrudes from the main body in a vehicle width direction indicating a width direction of the vehicle, the sensing portion is disposed at the protrusion portion, and a windbreak plate extending in the vehicle width direction and configured to prevent the airflow AC from directly hitting the sensing portion, said vehicle width direction is an intersection direction intersecting the airflow AC. a casing installed on the vehicle and defining an indoor chamber, the indoor chamber communicating with a vehicular compartment of the vehicle; an indoor heat exchanger arranged in the indoor chamber and having an interior for flow therethrough of a refrigerant containing carbon dioxide; a cooperation device (110a, 110b, 120a, 120b, 140a, 140b, 150a, 150b) included in, together with the indoor heat exchanger, a refrigeration cycle device for circulation therethrough of the refrigerant; the cooperation device (110a, 110b, 120a, 120b, 140a, 140b, 150a, 150b) comprising a compressor (110a;110b), an outdoor heat exchanger (120a;120b), an expander, an accumulator (140a; 140b) and a refrigerant piping (150a; 150b); an indoor fan arranged in the indoor chamber and configured to form airflow, the airflow being formed by air flowing from the vehicular compartment into the indoor chamber, passing through the indoor heat exchanger, and flowing out to return to the vehicular compartment; and a first gas sensor to detect a concentration of carbon dioxide gas in a path of the airflow at a position located downstream of the indoor heat exchanger, wherein the first gas sensor includes: a sensing portion configured to react in accordance with the concentration of the carbon dioxide gas, a main body fixed to the casing; a protrusion portion that protrudes from the main body in a vehicle width direction indicating a width direction of the vehicle, the sensing portion is disposed at the protrusion portion, and a windbreak plate extending in the vehicle width direction and configured to prevent the airflow AC from directly hitting the sensing portion,