Method for producing glass substrate for magnetic disk and method for manufacturing magnetic disk

A method of making a glass substrate for a magnetic disk, comprising a mirror polishing step which includes contacting a polishing pad with a surface of a multi-component glass substrate, supplying a polishing liquid containing polishing grains to the surface of said glass substrate, and relatively moving said glass substrate and said polishing pad to polish the surface of said glass substrate, said magnetic disk allowing a magnetic head to fly at a floating height of 8 nm or less, wherein:

• colloidal silica grains having a grain size of not larger than 80 nm are used as the polishing grains;
• said polishing step being carried out under the conditions that the polishing liquid contains an inorganic acid and a buffer agent for preventing change in pH of the polishing liquid, that the pH value of the polishing liquid is kept at 3.0 or less, and that a zeta potential of said polishing grains contained in the polishing liquid is not higher than −24.7 mV, in order to control a degree of coagulation or dispersion in said polishing liquid so that a Duboff value which shapes an end contour of the resulting glass substrate obtained by the mirror polishing step falls within ±10 nm.