A gas separation device comprising: an adsorption tower that has an adsorbent to adsorb oxygen in a prescribed pressure and temperature environment, at least part of the adsorption tower being exposed to an atmosphere at a temperature higher than a normal temperature; a first feed passage connected to the adsorption tower, and configured to introduce into the adsorption tower a mixed gas sent from a blower device, the mixed gas containing nitrogen and oxygen as main components thereof, and a second feed passage connected to the adsorption tower and configured to introduce the mixed gas into the adsorption tower at a flow rate smaller than the first feed passage; a separated gas discharge passage connected to the adsorption tower and configured to discharge a separated gas from the adsorption tower, the separated gas containing nitrogen as a main component thereof, the separated gas being produced upon removal of oxygen, which is adsorbed by the adsorbent, from the mixed gas introduced from the first feed passage and the second feed passage; a first heat exchanging unit configured to cause heat exchange between the separated gas discharged from the adsorption tower and the mixed gas introduced to the adsorption tower from the first feed passage; an oxygen discharge unit configured to reduce an inner pressure of the adsorption tower, cause oxygen to desorb from the adsorbent, and discharge oxygen from the adsorption tower; and a second heat exchanging unit configured to cause heat exchange between the desorbed oxygen and the mixed gas introduced to the adsorption tower from the second feed passage. an adsorption tower that has an adsorbent to adsorb oxygen in a prescribed pressure and temperature environment, at least part of the adsorption tower being exposed to an atmosphere at a temperature higher than a normal temperature; a first feed passage connected to the adsorption tower, and configured to introduce into the adsorption tower a mixed gas sent from a blower device, the mixed gas containing nitrogen and oxygen as main components thereof, and a second feed passage connected to the adsorption tower and configured to introduce the mixed gas into the adsorption tower at a flow rate smaller than the first feed passage; a separated gas discharge passage connected to the adsorption tower and configured to discharge a separated gas from the adsorption tower, the separated gas containing nitrogen as a main component thereof, the separated gas being produced upon removal of oxygen, which is adsorbed by the adsorbent, from the mixed gas introduced from the first feed passage and the second feed passage; a first heat exchanging unit configured to cause heat exchange between the separated gas discharged from the adsorption tower and the mixed gas introduced to the adsorption tower from the first feed passage; an oxygen discharge unit configured to reduce an inner pressure of the adsorption tower, cause oxygen to desorb from the adsorbent, and discharge oxygen from the adsorption tower; and a second heat exchanging unit configured to cause heat exchange between the desorbed oxygen and the mixed gas introduced to the adsorption tower from the second feed passage.