An energy-dispersion X-ray analyzer (EDX) attached to a scanning electron microscope was used to evaluate the distribution of lubricant on the surface of tablets prepared by using an external lubrication system (ELS). Optimal operating conditions for EDX were determined (probe current: 0.40 nA, accelerating voltage: 7.0 kV, and measurement time: 200 sec). Quantitative analysis was found to be possible by using a tablet prepared by the conventional method with magnesium stearate (Mg-St) and determining the amount of Mg detected by EDX. In the application of EDX to ELS, an increase in charged voltage brought about an increase of Mg sensing efficiency on the tablet surface. Further, Mg was not distributed uniformly on the tablet surfaces, with more Mg observed on the punch-side surface than on the die-side surface. These findings indicate that EDX is applicable to investigating the distribution of small quantities of Mg-St and is one of the useful methods for formulation studies based on “Quality by Design