The present invention provides glass containing, in terms of the oxide-based molar percentage, 60-75% of SiO2, 1-7% of Al2O3, 8-16% of MgO, 0-3.3% of CaO, 12-18% of Na2O, and 0-4% of K2O, and exhibiting a change in refractive index of 0.001 or less when heated to or above the annealing point and slowly cooled at a rate of 1DEG C/min. The glass of the invention makes it possible to reduce deformation in heat-treatment steps such as chemical strengthening and film-formation treatment in the actual production line.