The gas separation device includes an adsorption tower having at least one part thereof exposed to an atmosphere at a higher or lower temperature than normal temperature, a mixed gas feed unit, an adsorbent provided inside the adsorption tower to adsorb a matter contained in a mixed gas upon contact with the mixed gas in a prescribed pressure and temperature environment, and separate the matter from the mixed gas, a separated gas discharge unit that discharges a separated gas from the adsorption tower, and an adsorbed gas discharge unit that discharges from the adsorption tower the adsorbed gas which is adsorbed by the adsorbent. Heat reserving elements are arranged in the adsorption tower at positions upstream and downstream of the adsorbent in the mixed gas supply direction respectively such that the mixed gas, separated gas, and adsorbed gas flow through the heat reserving elements.