Experimental studies of the etching of platinum with a SiO2 etching mask in an Ar/O-2 mixed gas plasma were performed. The etching selectivity of platinum to SiO2 increases with the addition of oxygen, and a high etching selectivity of more than 6 is obtained around an oxygen concentration of 10%. This high etching selectivity is caused by the difference in the adsorption of oxygen atoms between platinum and SiO2. In the etching of SiO2 by an Ar/O-2 plasma, oxygen atoms are removed by sputtering by incident ions; however, these vacant sites of oxygen are filled immediately by the oxygen atoms incident from the plasma. The surface etching of SiO2 in an Ar/O-2 plasma is disturbed by these replacements of the oxygen atoms. On a platinum surface incident oxygen atoms do not stay on the surface, therefore, disturbance of the etching by oxygen adsorption does not occur, This difference in the oxygen adsorption between platinum and SiO2 is confirmed by the x-ray photoelectron spectroscopy analysis of the etched surfaces. By using an Ar/O-2 plasma where a high etching selectivity of Pt/SiO2 is obtained, platinum can be etched with thin SiO, etching masks. By optimizing both the thickness of the SiO2 mask and the etching time, platinum can be etched without any residue from the redeposition of the etching products.