To provide a process and an apparatus for treating a waste anesthetic gas containing a volatile anesthetic and nitrous oxide discharged from an operating room by introducing the gas into an adsorbing cylinder filled with an adsorbent, where the volatile anesthetic contained in the waste anesthetic gas is adsorbed and thereby removed, and successively introducing the gas into a catalyst layer filled with a nitrous oxide decomposition catalyst, where nitrous oxide is decomposed into nitrogen and oxygen. By using the process and the apparatus for treating a waste anesthetic gas of the present invention, a volatile anesthetic having a possibility of destroying the ozone layer or nitrous oxide as a global warming gas can be made harmless while preventing the release into atmosphere.