Compositional characterization of inclusion/matrix boundaries and relation to pit initiation at MnS in stainless steel were investigated by microelectrochemical measurements. In 0.1 M NaCl solution, N-containing 304L stainless steel (0.12 mass% N), MnS dissolution did not occurred. The thin-film layer was found at MnS/steel boundaries. Cr and C accumulated in the thin-film layer. The transpassive dissolution potential of Cr is considered to be more positive than dissolution potential of MnS. Since MnS was covered with the thin-film layer, it is thought that MnS did not dissolve. MnS inclusions have the solubility of Cr, so that (Mn, Cr)S was formed in the solidification process. It is thought that when Cr diffuse from (Mn, Cr)S to steel matrix during heattreatments, the thin-film layer is formed. About C, since there is almost no signal of C in inclusions before polarization, C is considered to be accumulated in the thin-film layer during the corrosion.