A subject of a tempered glass of the invention is to provide a tempered glass and a glass for tempering in which density and high temperature viscosity are decreased, a KNO3 melting salt is not easilydeteriorated, and thermal shock resistance is excellent. The tempered glass of the invention has a compression stress layer on a surface and is characterized by: as a glass composition, containing SiO2 ranging from 50% to 80%, Al2O3 ranging from 5% to 30%, Li2O ranging from 0% to 2%, Na2O ranging from 5% to 25%, and K2O ranging from 0% to 5% in mol%, and not containing As2O3, Sb2O3, PbO, and F substantially.本發明的強化玻璃的課題為提出降低密度與高溫黏度，並且使KNO3熔融鹽不易劣化，且耐熱衝擊性優異的強化玻璃以及強化用玻璃，本發明的強化玻璃在表面具有壓縮應力層，其特徵在於：作為玻璃組成，以莫耳%計含有50%~80%的SiO2、5%~30%的Al2O3、0%~2%的Li2O、5%~25%的Na2O、以及0%~5%的K2O，且實質上不含有As2O3、Sb2O3、PbO、以及F。
