Processes for removing N.sub.2 from a feed gas comprising CO+N.sub.2 or CO, CO.sub.2 +N.sub.2 through PSA by using at least two adsorption columns containing an adsorbent exhibiting selective adsorb property to carbon monoxide which comprises:(i) a step of pressurizing an adsorption column by the feed gas;(ii) a step of introducing the feed gas into the adsorption column, in which step (i) was previously completed, so as to adsorb CO, or CO+CO.sub.2 on or in the adsorbent;(iii) a step of connecting the adsorption column, in which step (ii) was previously completed, to the other adsorption column in which step (v) was previously completed, to reduce the pressure in the former adsorption column to one atmosphere or a pressure close to it;(iv) a step of purging nitrogen by passing product gas through the adsorption column;(v) a step of desorbing carbon monoxide adsorbed on or in the adsorbent of the adsorption column, by vacuum pump to recover a product gas; and(vi) a step of a connecting the adsorption column, in which step (v) was previously completed, to the other adsorption column in which step (ii) was previously completed to increase pressure in the former column,periodically switching the flow between or among said adsorption columns so as to repeat the above steps in all the adsorption columns.