A gas supply device provided for a container in which a breathing plant is housed, the device comprising: a first adsorption portion and a second adsorption portion in each of which an adsorbent for adsorbing nitrogen in air is provided; an air pump having a first pump mechanism connected to one of the first adsorption portion and the second adsorption portion, and a second pump mechanism connected to the other one of the first adsorption portion and the second adsorption portion, the first pump mechanism supplying outside air to the first or second adsorption portion to allow the adsorbent to adsorb nitrogen in the outside air and thereby producing oxygen-enriched air having a higher oxygen concentration than the outside air, the second pump mechanism sucking air from the first or second adsorption portion and thereby producing nitrogen-enriched air containing the nitrogen desorbed from the adsorbent; a motor which drives the air pump; a switching mechanism (32, 33) which alternately switches between a first connection state in which the first adsorption portion is connected to the first pump mechanism and the second adsorption portion is connected to the second pump mechanism, and a second connection state in which the second adsorption portion is connected to the first pump mechanism and the first adsorption portion is connected to the second pump mechanism; a supply passage which allows the second pump mechanism to communicate with an interior of the container and guides the nitrogen-enriched air to the interior of the container; and an oxygen discharge passage which allows the first and second adsorption portions and to communicate with an outside and guides the oxygen-enriched air to the outside, wherein the gas supply device has a malfunction diagnosis section which checks whether each of components of the gas supply 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, and the malfunction diagnosis section checks whether each of the air pump, the motor, and the switching mechanism (32, 33) works properly or not, and determines that the adsorbent is not working properly when it is determined that all of the air pump, the motor, and the switching mechanism (32, 33) work properly. a first adsorption portion and a second adsorption portion in each of which an adsorbent for adsorbing nitrogen in air is provided; an air pump having a first pump mechanism connected to one of the first adsorption portion and the second adsorption portion, and a second pump mechanism connected to the other one of the first adsorption portion and the second adsorption portion, the first pump mechanism supplying outside air to the first or second adsorption portion to allow the adsorbent to adsorb nitrogen in the outside air and thereby producing oxygen-enriched air having a higher oxygen concentration than the outside air, the second pump mechanism sucking air from the first or second adsorption portion and thereby producing nitrogen-enriched air containing the nitrogen desorbed from the adsorbent; a motor which drives the air pump; a switching mechanism (32, 33) which alternately switches between a first connection state in which the first adsorption portion is connected to the first pump mechanism and the second adsorption portion is connected to the second pump mechanism, and a second connection state in which the second adsorption portion is connected to the first pump mechanism and the first adsorption portion is connected to the second pump mechanism; a supply passage which allows the second pump mechanism to communicate with an interior of the container and guides the nitrogen-enriched air to the interior of the container; and an oxygen discharge passage which allows the first and second adsorption portions and to communicate with an outside and guides the oxygen-enriched air to the outside, wherein the gas supply device has a malfunction diagnosis section which checks whether each of components of the gas supply 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, and the malfunction diagnosis section checks whether each of the air pump, the motor, and the switching mechanism (32, 33) works properly or not, and determines that the adsorbent is not working properly when it is determined that all of the air pump, the motor, and the switching mechanism (32, 33) work properly.