After a polishing process of polishing a glass substrate with an abrasive containing lanthanoid oxides, the glass substrate is subjected to the first and second washing processes. In the first washing process, the polished substrate is washed with a washing solution containing acid and a reducing agent, wherein the acid includes at least nitric acid. In the second washing process, the washed substrate is treated with an aqueous solution of an alkaline detergent. The substrate is suitable for a recording medium.