A glass substrate used as a substrate of an information recording medium such as a magnetic disk, magneto-optical disk, DVD, or MD, and a glass composition used to make such a glass substrate, contains the following glass ingredients: 40 to 70% by weight of SiO.sub.2; 1 to 20% by weight of Al.sub.2O.sub.3; 0 to 10% by weight, zero inclusive, of B.sub.2O.sub.3; SiO.sub.2+Al.sub.2O.sub.3+B.sub.2O.sub.3 accounting for 60 to 90% by weight; a total of 3.0 to 15% by weight of R.sub.2O compounds, where R=Li, Na, and K; a total of 2.0 to 15% by weight of R'O compounds, where R=Mg, and Zn; and a total of 1.0 to 20% by weight of MOx (TiO.sub.2+ZrO.sub.2+Ln.sub.xO.sub.y), where Ln.sub.xO.sub.y represents at least one compound selected from the group consisting of lanthanoid metal oxides, Y.sub.2O.sub.3, Nb.sub.2O.sub.5, and Ta.sub.2O.sub.5. Here, the following condition is fulfilled: 0.070<(total content of R'O compounds)/(SiO.sub.2+Al.sub.2O.sub.3+B.sub.2O.sub.3)<0.200.