We examined the feasibility of applying the glow discharge mass spectrometry (GD-MS) to the analysis of cast iron. Using 10 standard reference materials of cast iron, including NIST SRM, BS CRM, and BAS CRM, we obtained the relative sensitivity factors (RSFs) of C, Mg, Si, P, S, V, Cr, Mn, Ni, Cu, and Mo. We also performed comparisons with spark discharge optical emission spectrometry (SP-OES). Determination of alloying elements such as Si and Ni in cast iron by GD-MS revealed no bias among our study specimens, and the fluctuations of the RSFs in RSD were around 3%. For carbon, an important element, however, some specimens exhibited a low ion intensity ratio from GD-MS but a high emission intensity ratio from SP-OES. We suspect the cause of this lies in the metallic texture related to Fe and C, and the performed observation of EPMA. Therefore, we judged the cause when using GD-MS is the difference in ionization efficiency between graphite and matrix (cementite), or selective sputtering, and the cause when using SP-OES is abnormal discharge resulting from deposited graphite.