The invention is to provide a method in which waviness generated on a glass substrate surface during pre-polishing is removed, thereby finishing the glass substrate so as to have a surface excellent in flatness. The invention relates to a method for finishing a pre-polished glass substrate surface using any one of processing methods selected from the group consisting of ion beam etching, gas cluster ion beam etching and plasma etching, the glass substrate being made of quartz glass that contains a dopant and comprises SiO.sub.2 as a main component, and the method for finishing a pre-polished glass substrate surface including: a step of measuring flatness of the glass substrate surface using a shape measurement unit that comprises: a low-coherent light source whose outgoing light flux has a coherence length shorter than twice an optical distance between front and back surfaces of the glass substrate; a path match route part that divides the outgoing light flux from the low-coherent light source into two light fluxes, causes one of the two light fluxes to make a detour by a given optical path length relative to the other light flux, and then recombines the light fluxes into a single light flux and outputs it; and an interference optical system that acquires an interference fringe which carries wave surface information of the glass substrate surface by radiating an outgoing light flux from the low-coherent light source onto a reference surface and the glass substrate surface held on a measurement optical axis and making lights returning from the reference surface and the glass substrate surface interfere with each other; and a step of measuring a concentration distribution of the dopant contained in the glass substrate, wherein processing conditions of the glass substrate surface are set up for each site of the glass substrate based on the results obtained from the step of measuring flatness of the glass substrate and the step of measuring a concentration distribution of the dopant contained in the glass substrate, and the finishing is carried out while keeping an angle formed by a normal line of the glass substrate and an incident beam onto the glass substrate surface at from 30 to 89.degree..