A glass substrate for a magnetic disk, wherein both side surface layer portions are formed in compressive stress layers and an intermediate portion between these compressive stress layers is formed in a tensile stress layer by chemical reinforcement. Where the thickness of the substrate is less than 0.5 mm, the thickness of the tensile stress layer is L, and the value of a tensile stress in the tensile stress layer is Pt [kg/mm<2>], the relation of 0.4 [kg/mm] <= L.Pt <= 2.0 [kg/mm] is established. |