This invention relates to a glass substrate having a high glass transition temperature, a thermal expansion coefficient near that of soda-lime glass, and a low specific gravity, which is not prone to yellowing, and which also has good solubility and high productivity. The glass substrate has a composition in mass % in terms of oxide amount which is SiO2: 55-65%, Al2O3: 4-8%, MgO: 6-9%, CaO: 0.1-5%, SrO: 0.5-6%, BaO: 0-2%, MgO+CaO+SrO+BaO: 6.6-19%, Na2O: 0-5%, K2O: 9.5-21%, Na2O+K2O: 10-22%, ZrO2: 0.5-5%, and Fe2O3: 0.06-0.15%, has a specific gravity of 2.7 or less, an average thermal expansion coefficient at 50-350° C. of 80×10−7/° C. to 90×10−7/° C., and a glass transition temperature of at least 640° C., wherein, defining η as viscosity, the temperature satisfying log η=2 is at most 1550° C., and the yellow coloring b* of the glass surface is at most 8.