A supporting glass substrate has a ratio of a Young's modulus (GPa) to a density (g/cm3) that is 37.0 (GPa·cm3/g) or more and the ratio has a value larger than a ratio calculation value, the ratio calculation value being a ratio of a Young's modulus (GPa) calculated from a composition to a density (g/cm3). The ratio calculation value is represented by the following expression: α=2·Σ{(Vi·Gi)/Mi·Xi}, where, in the expression, Vi is a filling parameter of a metal oxide contained in the supporting glass substrate, Gi is a dissociation energy of a metal oxide contained in the supporting glass substrate, Mi is a molecular weight of a metal oxide contained in the supporting glass substrate, and Xi is a molar ratio of a metal oxide contained in the supporting glass substrate.