The present invention relates to an alkali-free glass substrate with a thickness of 0.4 mm or less, which has been reduced in thickness by 5 µm or more by a hydrofluoric acid (HF) etching treatment, in which the alkali-free glass substrate contains an alkali-free glass described below, and the alkali-free glass substrate after reduced in thickness has a specific modulus of 32 MNm/kg or more and a photoelastic constant of 31 nm/MPa/cm or less, the alkali-free glass having a strain point of 710°C or higher, an average thermal expansion coefficient at 50 to 350°C of from 30×10 -7 to 43×10 -7 /°C, a temperature T 2 at which a glass viscosity reaches 10 2 dPa·s of 1710°C or lower, a temperature T 4 at which the glass viscosity reaches 10 4 dPa·s of 1320°C or lower, and containing, indicated by mol% on the basis of oxides, SiO 2 66 to 70, Al 2 O 3 12 to 15, B 2 O 3 0 to 1.5, MgO exceeding 9.5 to 13, CaO 4 to 9, SrO 0.5 to 4.5, BaO 0 to 1, and ZrO 2 0 to 2, in which MgO+CaO+SrO+BaO is from 17 to 21, MgO/(MgO+CaO+SrO+BaO) is 0.40 or more, MgO/(MgO+CaO) is 0.40 or more, and MgO/(MgO+SrO) is 0.60 or more.