A synthetic quartz glass substrate is machined by bringing a surface of the synthetic quartz glass substrate as the workpiece into contact with and superposing it on a surface of a protective member made of synthetic quartz glass to effect optical contact bonding of the workpiece and the protective member, and passing a cutting tool through the optical contact bonding surfaces. This machining process is able to effectively prevent the generation of microdefects at the cutting tool entry site and extraction site during a cutting operation. Moreover, a fixing agent is not used to join the workpiece and the protective member, and so productivity is high because there is no need for the application and later removal of a fixing agent.