A process for preparing an azetidinone derivative represented by the formula ##STR1## wherein R.sup.1 represents a straight-chain or branched-chain lower alkyl group, substituted or unsubstituted aryl group, substituted or unsubstituted phenylmethyl group or substituted or unsubstituted phenyloxymethyl group, R.sup.2 represents hydrogen atom, an optionally substituted hydrocarbon residue or protective group for amino group selected from the class consisting of acyl, silyl, sulfonyl and phosphonyl derived from organic or inorganic acid, and R.sup.3 represents a substituted or unsubstituted aryl group or the residue of substituted or unsubstituted heterocyclic ring.