A glass substrate for use as the substrate of an information recording medium such as a magnetic disk, magneto-optical disk, DVD, or MD or of an optical communication device, and a glass composition for making such a glass substrate, contains the following glass ingredients: 45 to 75% by weight of SiO.sub.2; 1 to 20% by weight of Al.sub.2O.sub.3; 0 to 15% 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 65 to 90% by weight; a total of 7 to 20% by weight of R.sub.2O compounds, where R=Li, Na, and K; and a total of 0 to 12% by weight, zero inclusive, of R'O compounds, where R'=Mg, Ca, Sr, Ba, and Zn. Moreover, the following conditions are fulfilled: B.sub.2O.sub.3=0% by weight, or Al.sub.2O.sub.3/B.sub.2O.sub.3.gtoreq.1.0; and (SiO.sub.2+Al.sub.2O.sub.3+B.sub.2O.sub.3)/(the total of R.sub.2O compounds+the total of R.sub.2O compounds).gtoreq.3.