A container refrigeration apparatus provided in a container , having a refrigerant circuit where a refrigerant circulates to perform a refrigeration cycle, and allowing heat exchange in an evaporator connected to the refrigerant circuit between air in the container and the refrigerant to cool the air, wherein the container houses a fan producing an air flow that passes the evaporator and circulates in the container, and an air passage having one end opened on a blowout side of the fan and the other end opened on a suction side of the fan so that the air in the container flows from the one end to the other end of the air passage as the fan rotates, the container refrigeration apparatus comprising: a gas supply device which produces nitrogen-enriched air having a higher nitrogen concentration than outside air and supplies the nitrogen-enriched air to the interior of the container via a supply passage ; an oxygen concentration sensor which is provided for the air passage to measure the oxygen concentration of the air passing through the air passage in the container ; a controller which controls the operation of the gas supply device so that the oxygen concentration measured by the oxygen concentration sensor reaches a target concentration; a measurement passage , which is comprised of a branch pipe which connects the supply passage and the air passage , an on-off valve provided for the measurement passage , characterized in that the branch pipe is configured to diverge, and guide to the air passage , part of the nitrogen-enriched air passing through the supply passage to the oxygen concentration sensor , so that the oxygen concentration sensor measures the oxygen concentration of the nitrogen-enriched air, and the controller is configured to perform, while the gas supply device is in operation and a rotation of the fan is stopped, a supply air measurement operation in which the on-off valve is opened and the oxygen concentration sensor is allowed to measure the oxygen concentration of the nitrogen-enriched air. a gas supply device which produces nitrogen-enriched air having a higher nitrogen concentration than outside air and supplies the nitrogen-enriched air to the interior of the container via a supply passage ; an oxygen concentration sensor which is provided for the air passage to measure the oxygen concentration of the air passing through the air passage in the container ; a controller which controls the operation of the gas supply device so that the oxygen concentration measured by the oxygen concentration sensor reaches a target concentration; a measurement passage , which is comprised of a branch pipe which connects the supply passage and the air passage , an on-off valve provided for the measurement passage , characterized in that the branch pipe is configured to diverge, and guide to the air passage , part of the nitrogen-enriched air passing through the supply passage to the oxygen concentration sensor , so that the oxygen concentration sensor measures the oxygen concentration of the nitrogen-enriched air, and the controller is configured to perform, while the gas supply device is in operation and a rotation of the fan is stopped, a supply air measurement operation in which the on-off valve is opened and the oxygen concentration sensor is allowed to measure the oxygen concentration of the nitrogen-enriched air. <EOS>