Disclosed is a glass composition which is adapted to use in providing a hermetic seal between a metal and a ceramic, a metal and a metal, and a ceramic and a ceramic, to form crystallized glass having high strength and high thermal expansion which allow it to be used at high temperatures of 950° C. and over. The glass composition is substantially free of alkali metals, and contains, as calculated as oxide, 15-30 mass % of SiO2, 0-5 mass % of Al2O3, 20-35 mass % of B2O3, 10-25 mass % of CaO, 25-40 mass % of MgO, 3-8 mass % (3.0% excluded) of ZrO2, and 0-3 mass % of CeO2, wherein the crystallized glass which is formed by firing a glass powder made of the glass composition at a temperature of 850-1050° C. has a thermal expansion coefficient of 90-110×10−7/° C. at 50-550° C. and flexural strength of not less than 80 MPa.