SUS304 stainless steel rods were oxidized at 1073 K in air to form oxide film and then immersed in NaOH solution to be polarized cathodically at a cell voltage of 125 V. The electrode emitted light due to the formation of a plasma state at the interface between the electrode surface and the solution caused by the strong electric field applied to the interface. Atomic emission lines of Fe, Cr and Ni were found in the light spectra. The intensity of light was stronger on the oxide layer than on the metal surface. From the time transient of intensities of light, the oxide layer was found to be almost removed from the surface after high-voltage polarization within 10 s. From comparison of experimental results with a model calculation assuming the sputtering of oxide film with bombardment by charged particles in the plasma phase, it was estimated that the removal of surface oxide layer was done by micro-arc discharge mechanism rather than continuous sputtering. Because the formation of micro-arc preferentially occurred on the oxide layer rather than on metal surface, the removal of oxide layer from the surface was considerably rapid. On the other hand, at the point where micro-arc discharge occurred, melting of metal surface was found. (c) 2007 Elsevier Ltd. All rights reserved.