Applying the characteristics of the emission spectral analysis for the purpose of local analysis, qualitative, of metallic structures, as the microscopic observation, the apparatus was built for catching the spectral lines which resulted from the spark between the specimen surface and the Pt-wire put into a small hole of a transparent quartz tube, a medium quartz spectrograph a metallic microscope and a transparent quartz tube were employed for this purpose. Although the transparent quartz tube and the photographic plate allowed an accuracy of only 0.08±0.02 mm at the spark point, the apparatus was sufficient to achieve the sought result. The apparatus was applied to Mg analysis for Mg nodular cast iron that at any degree of spheroidization of nodular cast iron the Mg content in the spheroidal graphite was larger than that in the matrix and when the analysis value of Mg contained in the nodular cast iron was larger than about 0.08% (segregation not considered), the matrix was supposed to be containing Mg.