Disclosed is a gas supply device provided for a container in which a breathing plant is housed. The device supplies nitrogen-enriched air into the container. The device is provided with a malfunction diagnosis section which checks whether each of components of the device works properly or not and specifies a malfunctioning portion if an oxygen concentration of the nitrogen-enriched air does not fall to or below a predetermined concentration. The malfunction diagnosis section checks whether each of an air pump, a motor, and a switching mechanism works properly or not, and determines that an adsorbent is not working properly when it is determined that all of the air pump, the motor, and the switching mechanism are working properly.