The present invention aims at providing a glass substrate required to have a surface polished with extremely high accuracy as in glass substrates for reflective masks for use in EUVL; and a polishing method for producing the glass substrate. The present invention provides a glass substrate for mask blank, which is a glass substrate comprising SiO.sub.2 as a main component and having a polished main surface, wherein concave defects and convex defects on the main surface have a depth of 2 nm or smaller and a height of 2 nm or smaller, respectively, and have a half-value width of 60 nm or smaller, so that the concave defects and/or the convex defects do not cause phase defects when the glass substrate is used to produce a mask for exposure and the mask is used. Also disclosed are a polishing method for producing the glass substrate, and a mask blank and a mask for exposure using the glass substrate.