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. In the present invention, a waste anesthetic gas containing a volatile anesthetic and nitrous oxide is introduced 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 this gas is introduced into a catalyst layer filled with a nitrous oxide decomposition catalyst, where nitrous oxide is decomposed into nitrogen and oxygen.