To provide a polishing method for a glass substrate required to have extremely high surface smoothness and surface precision, like a glass substrate to be used for e.g. a reflective mask for extreme ultra violet lithography. A surface of a glass substrate containing SiO.sub.2 as the main component, is polished with a polishing slurry comprising colloidal silica having an average primary particle size of at most 50 nm, an acid and water, and having the pH adjusted to be within a range of from 0.5 to 4, so that the surface roughness Rms will be at most 0.15 nm as measured by an atomic force microscope.