a process for producing a 4 - acetoxyazetidinone represented by formula : ## str1 ## wherein z represents a hydrogen atom , a lower alkyl group , or a protected or unprotected hydroxyethyl group ; and w represents a hydrogen atom , a lower alkyl group , or a group of -- coor 1 , wherein r 1 represents a lower alkyl group , is disclosed , which comprises reacting an azetidinone represented by formula : ## str2 ## wherein z is as defined above ; and y represents a hydrogen atom , carboxyl group , a lower alkyl group , or a group of -- coor 1 , wherein r 1 represents a lower alkyl group , with acetic acid and an oxidizing agent in the presence of , as a catalyst , an anhydrous or hydrous osmium compound represented by osx 3 , wherein x represents a chlorine atom , a bromine atom , or an iodine atom .