A process for preparing an azetidinone derivative represented by the formula ##STR1## wherein R.sup.1 is hydrogen, halogen or lower alkoxy, R.sup.2 is hydrogen, halogen, lower alkoxy, amino or a group ##STR2## (in which R.sup.5 is substituted or unsubstituted phenyl, substituted or unsubstituted phenylmethyl, substituted or unsubstituted phenoxymethyl, or substituted or unsubstituted benzoyl), or R.sup.1 and R.sup.2, when taken together, are carbonyl, R.sup.3 is substituted or unsubstituted phenyl, and R.sup.4 is hydrogen, optionally substituted hydrocarbon residue or acyl, silyl, sulfonyl or phosphonyl derived from inorganic acid or organic acid, the process comprising reacting a dithioazetidinone derivative represented by the formula ##STR3## wherein R.sup.1, R.sup.2 and R.sup.4 are as defined above and R.sup.9 is substituted or unsubstituted, nitrogen-containing aromatic heterocyclic residue with a compound represented by the formulaR.sup.3 SO.sub.2 H (VII)wherein R.sup.3 is defined above.