NOVELTY - Ammonia and nitrogen oxide as impurity components in mixed gas containing krypton, xenon and/or neon are removed by reacting mixed gas with nitrogen-oxide removal catalyst. Nitrogen oxide in mixed gas is reduced. The mixed gas is contacted with an adsorption agent (a) and adsorption agent (b). The recovered gas component is contacted with adsorption agent (c) using krypton, xenon and neon as difficulty-adsorbable gas component. The easily-adsorbable gas component is adsorption-separated with adsorption agent (c).    USE - Separation of impurity component in mixed gas discharged during oxidation process, nitridation process and oxynitridation process in manufacture of semiconductor apparatus e.g. integrated circuit, liquid crystal panel, solar cell and magnetic disk.    ADVANTAGE - The method reliably, efficiently and economically removes nitrogen oxide, ammonia, oxygen, nitrogen, hydrogen, helium and steam in the mixed gas.    DETAILED DESCRIPTION - Ammonia and nitrogen oxide as impurity components in mixed gas containing krypton, xenon and/or neon are removed by reacting mixed gas with nitrogen-oxide removal catalyst, and converted to nitrogen and steam. Denitration is carried out to convert nitrogen oxide in mixed gas to nitrogen and steam in presence of a reducing substance. The mixed gas is contacted with an adsorption agent (a), using ammonia and steam as easily-adsorbable gas component, and ammonia and steam are adsorbed to the adsorption agent (a) and separated from the mixed gas. The mixed gas is contacted with adsorption agent (b) using krypton, xenon and neon as easily-adsorbable gas component and the difficulty-adsorbable impurity component with respect to adsorption agent (b). The gas component which adsorbed to the adsorption agent (b) is removed from the adsorption agent (b). The recovered gas component is contacted with adsorption agent (c) using krypton, xenon and neon as difficulty-adsorbable gas component. The easily-adsorbable gas component is adsorption-separated with respect to adsorption agent (c). An INDEPENDENT CLAIM is included for apparatus for separating impurity component in mixed gas, which has catalytic reaction unit to remove ammonia and/or nitrogen oxide, nitrogen-oxide removal catalyst reaction unit (21) filled with nitrogen-oxide removal catalyst and which converts nitrogen oxide discharged from exit of catalytic reaction unit to nitrogen and steam in presence of reducing substance, adsorption separation unit (31) filled with adsorption agent (a), adsorption separation unit (41) filled with adsorption agent (b), reducing substance addition path (23) connected to the entrance side of nitrogen-oxide removal catalyst reaction unit, introduction path (32) to introduce gas discharged from exit of nitrogen-oxide removal catalyst reaction unit to entrance side of adsorption column (c1), introduction path (42) to introduce gas component which is not adsorbed to the adsorption agent (a) discharged from exit of adsorption column (c1) to entrance of adsorption separation unit (41), lead-out passage (p1) which leads out gas component which is not adsorbed to the adsorption agent (b) discharged from exit of adsorption separation unit (41), recovery path (47) to recover gas component which is adsorbed to the adsorption agent (b), adsorption separation unit (51) filed with adsorption agent (c), introduction path (52) which introduces gas component recovered from adsorption separation unit (41) to entrance of adsorption separation unit (51) and lead-out passage (p2) which leads-out gas component which is not adsorbed to the adsorption agent (c) from exit of adsorption column (c3).    DESCRIPTION OF DRAWING(S) - The drawing shows the schematic view of the gas-separation apparatus.    Nitrogen oxide removal catalyst reaction unit (21)    Reducing substance addition path (23)    Adsorption separation units (31,41,51)    Introduction paths (32,42,52)    Recovery path (47)