In the table-salt manufacturing process, the mother liquid and the concentrated brine in a crystallizer are highly concentrated salt-mixture solutions comprising NaCl, MgCl₂, KCl and CaCl₂, and a determination of their concentrations plays an important role. A method is proposed to determine the concentration of each component in the mother liquid and the concentrated brine using the absorbance change relative to the water absorption at six wave numbers by attenuated total-reflectance IR (ATR-IR) spectrometry; an experimental confirmation can be used to accurately determine the composition. In particular, for MgCl₂, whose spectral change is larger than that of the other components, its concentration can be quantitatively determined with an accuracy nearly equal to that of a chemical analysis.