The present invention is to provide a processing method for manufacturing a highly flat and highly smooth glass substrate with good productivity. A highly flat and highly smooth glass substrate is obtained with good productivity by processing of a glass substrate, which comprises a step of measuring the surface shape of the glass substrate prior to processing, a step of processing the surface of the substrate by changing a processing condition for each site (first processing step), and a step of finish-polishing the surface of the glass substrate that has been subjected to the first processing step (second processing step). At that time, the processing condition for each site within the surface of the substrate in the first processing step is determined from a processing amount that is determined from the concave-convex shape of the surface of the glass substrate prior to processing and the in-plane distribution of a processing amount by the second processing step separately measured by using a similar substrate.