A method for manufacturing a glass substrate according to which surface roughnesses of main surfaces of a glass substrate can be reduced more than with currently available methods is provided. After the main surfaces of the glass substrate used in a magnetic disk are mirror-polished (final finishing-polished) using a polishing liquid containing organic-based particles made of, for example, a styrene-based resin, an acrylic resin, or a urethane-based resin as polishing abrasive particles, by cleaning the glass substrate using an organic-based cleaning agent, surface roughnesses of the main surfaces of the substrate can be reduced more than with currently available methods.