An azetidinone derivative represented by the formula (I) ##STR1## wherein R.sup.1 is substituted or unsubstituted phenyl or substituted or unsubstituted phenoxy, R.sup.2 is hydrogen, optionally substituted hydrocarbon residue or aminoprotecting group selected from acyl, silyl, sulfonyl and phosphonyl derived from organic or inorganic acid, ##STR2## (wherein R.sup.3 is hydrogen or lower alkyl), X.sup.1 and X.sup.2 are the same or different and are halogen, hydroxyl, alkoxy, acyloxy, SR.sup.4 (wherein R.sup.4 is straight chain or branched chain lower alkyl substituted or unsubstituted, substituted or unsubstituted phenyl or substituted or unsubstituted heterocyclic group), amino or hydrogen, one of X.sup.1 and X.sup.2 being hydrogen when the other is not, and R.sup.5 is substituted or unsubstituted phenyl or substituted or unsubstituted heterocyclic group, and a process for preparing the same.