A container refrigeration apparatus attachable to a container configured to house a breathing plant, the apparatus comprising: a refrigerant circuit to which an evaporator that is configured to allow air in the container to exchange heat with a refrigerant is connected, and in which the refrigerant is circulatable to perform a refrigeration cycle; and an inside air control system which has: a gas supply device having a supply passage which communicates with an inside of the container and through which supply air is suppliable into the container; and an oxygen sensor which is configured to measure an oxygen concentration of the air in the container, the inside air control system is configured to control the oxygen concentration of the air in the container to be a target concentration, wherein the inside air control system further has a branch pipe which is connected, at one end, to the supply passage and, at the other end, coupled to the oxygen sensor, and configured to guide at least part of the supply air flowing through the supply passage to the oxygen sensor, and an open/close valve which is configured to open/close the branch pipe, characterized in that a supply state of the gas supply device is switchable between a first supply state of producing, from outside air, nitrogen-enriched air having a higher nitrogen concentration and lower oxygen concentration than the outside air, and supplying, as the supply air, the nitrogen-enriched air into the container via the supply passage, and a second supply state in which the gas supply device is configured to take outside air therein and supply, as the supply air, the outside air into the container via the supply passage, and the inside air control system is configured to perform an outside air calibration operation in which the open/close valve is opened in the second supply state, and the at least part of the outside air flowing through the supply passage is guided to the oxygen sensor to calibrate the oxygen sensor using the outside air. a refrigerant circuit to which an evaporator that is configured to allow air in the container to exchange heat with a refrigerant is connected, and in which the refrigerant is circulatable to perform a refrigeration cycle; and an inside air control system which has: a gas supply device having a supply passage which communicates with an inside of the container and through which supply air is suppliable into the container; and an oxygen sensor which is configured to measure an oxygen concentration of the air in the container, the inside air control system is configured to control the oxygen concentration of the air in the container to be a target concentration, wherein the inside air control system further has a branch pipe which is connected, at one end, to the supply passage and, at the other end, coupled to the oxygen sensor, and configured to guide at least part of the supply air flowing through the supply passage to the oxygen sensor, and an open/close valve which is configured to open/close the branch pipe, characterized in that a branch pipe which is connected, at one end, to the supply passage and, at the other end, coupled to the oxygen sensor, and configured to guide at least part of the supply air flowing through the supply passage to the oxygen sensor, and an open/close valve which is configured to open/close the branch pipe, characterized in that a supply state of the gas supply device is switchable between a first supply state of producing, from outside air, nitrogen-enriched air having a higher nitrogen concentration and lower oxygen concentration than the outside air, and supplying, as the supply air, the nitrogen-enriched air into the container via the supply passage, and a second supply state in which the gas supply device is configured to take outside air therein and supply, as the supply air, the outside air into the container via the supply passage, and the inside air control system is configured to perform an outside air calibration operation in which the open/close valve is opened in the second supply state, and the at least part of the outside air flowing through the supply passage is guided to the oxygen sensor to calibrate the oxygen sensor using the outside air.