A gas supply device is configured such that its supply state is switchable between a first supply state supplying, as supply air, nitrogen-enriched air produced from outside air into a containe, and a second supply state in which the gas supply device takes outside air therein and supplies, as supply air, the outside air into the container. A CA system is provided with a branch pipe which guides at least part of the supply air flowing through a supply passage to an oxygen sensor, and an open/close valve which opens/closes the branch pipe. An outside air calibration operation is performed in which the open/close valve is opened in the second supply state to guide the least part of the outside air flowing through the supply passage to the oxygen sensor, thereby calibrating the oxygen sensor using the outside air.