An object of the invention is to obtain a glass substrate having high mechanical strength by reconciling suitability for ion exchange and devitrification proof in a glass. The strengthened glass substrate of the invention is a strengthened glass substrate having a compression stress layer in the surface thereof, the glass substrate having a glass composition including, in terms of % by mass, 40-70% of SiO2, 12-25% of Al2O3, 0-10% of B2O3, 0-8% of Li2O, 6-15% of Na2O, 0-10% of K2O, 13-20% of Li2O+Na2O+K2O, 0-3.9% of MgO, 0-5% of CaO, 0-5% of ZnO, 0-6% of ZrO2, and 0-5% of SrO+BaO, the value of (MgO+ZrO2+ZnO)/(MgO+ZrO2+ZnO+Al2O3) in terms of mass proportion being from 0.25 to 0.45. The above-mentioned strengthened glass can be produced by melting raw glass materials mixed together so as to result in the given glass composition, forming the melt into a sheet by an overflow downdraw process, and then conducting an ion exchange treatment to form a compression stress layer in the glass sheet surface.