To devise a tempered glass and a glass to be tempered each of which is lowered in density and viscosity at high temperature, hardly deteriorates a KNO3 molten salt, and is excellent in thermal shock resistance. The tempered glass having a compression stress layer in a surface thereof, including as a glass composition, in terms of mol %, 50 to 80% of SiO2, 5 to 30% of Al2O3, 0 to 2% of Li2O, 5 to 25% of Na2O, and 0 to 5% of K2O, and being substantially free of As2O3, Sb2O3, PbO, and F.
