An alkali-free glass substrate contains, as represented by mass % based on oxides: 54% to 66% of SiO2; 10% to 23% of Al2O3; 6% to 12% of B2O3; and 8% to 26% of MgO+CaO+SrO+BaO. The alkali-free glass substrate has β-OH of 0.15 mm−1 to 0.5 mm−1, and a Cl content of 0.1 to 0.35 mass %. A bubble growth index I of the alkali-free glass substrate given by the following formula is 280 or more: I=590.5×[β-OH]+874.1×[Cl]−5.7×[B2O3]−33.3. In the formula, [β-OH] is β-OH of the alkali-free glass substrate in mm−1, [Cl] is the Cl content of the alkali-free glass substrate in mass %, and [B2O3] is a B2O3 content of the alkali-free glass substrate in mass %.