A process for preparing an azetidinone derivative represented by the formula (1) ##STR1## wherein R.sup.1 represents a substituted or unsubstituted aryl group, a substituted or unsubstituted phenylmethyl group or a substituted or unsubstituted phenoxymethyl group, R.sup.2 represents a hydrogen atom or a group for protecting carboxylic acid and Ar represents a substituted or unsubstituted phenyl group, the process comprising reacting a thiazolineazetidinone derivative represented by the formula (2) ##STR2## wherein R.sup.1 and R.sup.2 are as defined above with a sulfonyl bromide represented by the formula (3)Ar--SO.sub.2 --Br (3)wherein Ar is as defined above in water-containing organic solvent.