To provide a method for manufacturing a synthetic quartz glass substrate for a polysilicon TFT (thin-film transistor) which suppresses the generation of impurities from a substrate end surface and suppresses the elution of the impurities from the end surface and adhesion thereof to the the surface of the substrate even if the synthetic quartz glass is cleaned with chemicals (an HF solution etc.) in a polishing process.

In the method for manufacturing the synthetic quartz glass substrate for the polysilicon TFT formed by multistage polishing consisting of a lapping process and a polishing process, the end surface of the substrate is subjected to mirror surface finishing just prior to the final polishing process of the polishing process and after subjecting the substrate to etching.