A gas supply device supplies an internal space of a container with nitrogen-enriched air to reduce an oxygen concentration of the internal space. If the oxygen concentration of the internal space gradually falls to reach a reference concentration C2, the gas supply device stops. If outside air enters the internal space at a gap between the container and a container refrigeration apparatus, the oxygen concentration of the internal space rises after the stop of the gas supply device. If the oxygen concentration of the internal space rises to reach an upper limit concentration C3, an internal fan controller switches a rotational speed of an internal fan from "HIGH" to "LOW." The gas supply device is activated. As a result, the amount of the outside air entering the internal space can be reduced to reliably drop the oxygen concentration of the internal space.
