Anodizing of magnesium was studied in sodium metasilicate with ortho-Cresol-4-sulfonic acid aqueous solution. A gray-colored anodic oxide film was obtained in 3.0mol·dm⁻³ sodium metasilicate-0.25mol·dm⁻³ o-Cresol-4-sulfonic acid aqueous solution at constant current density of 1000A·m⁻² for 6.0×10⁵C·m⁻² at 278K. This film mainly consisted of magnesium, silicon, and oxygen and contained small amounts of carbon and sulfur. In the wear test using #800 SiC waterproof abrasive paper, this anodic oxide film showed almost the same wear resistance as that obtained in 2.0mol·dm⁻³ sodium metasilicate aqueous solution. However, in 0.86mol·dm⁻³ NaCl solution at 308K, the former without sealing showed higher corrosion resistance than the latter with sealing. Moreover, its corrosion resistance was further improved by the sealing treatment in boiling distilled water.