In a method in which a cut line is formed on one surface of a planar glass material, and the cut line is allowed to extend in the thickness direction of the glass material, thereby cutting a glass substrate from the glass material, the cut line is selectively formed on a surface having relatively small surface waviness out of two opposing surfaces of the glass material. In the case of a glass material formed into a planar shape on a molten metal, the surface which has come into contact with the molten metal is selected as the surface having relatively small surface waviness. When a disk-shaped glass substrate is cut from the glass material, cutting is performed under conditions where either one of the thickness and the radius of the glass material and the maximum height of surface waviness of the glass material satisfy a predetermined relationship.