A process for producing a 4-substituted azetidinone derivative represented by the following general formula [III]: (wherein OR is a protected hydroxy group, Y is an alkyl group, an alkoxy group, a silyloxy group, a carbamoyloxy group, an amino group, a substituted or unsubstituted aromatic group or a substituted or unsubstituted heterocyclic group, and n is an integer of 0 or 1, provided that n does not represent O when Y is an alkoxy group, silyloxy group, carbamoyloxy group or amino group), characterized in that a 2-azetidinone derivative represented by the following general formula [I]: (wherein OR is as defined above, and X is an alkyl group or a substituted or unsubstituted aromatic group) is reacted with thiocarboxylic acid represented by the following general formula [II]: HSCO-(CH2)n-Y [II] (wherein Y and n are respectively as defined above) in an organic solvent in the presence of copper compounds. The aforementioned method can shorten the production process compared with the prior method. It is also highly advantageous from an industrial point of view because it does not employ mercury salts.