A ceramic substrate is provided as one having a large coefficient of thermal expansion α, having properties suitable for use as a high-frequency substrate, being capable of being fired at a low temperature, and having an excellent substrate strength. The ceramic substrate has a main composition containing Mg2SiO4 and a low-temperature-fired component, has the coefficient of thermal expansion a of not less than 9.0 ppm/° C., and contains up to 25 vol. % (excluding zero) ZnAl2O4 or up to 7 vol. % (excluding zero) Al2O3. A dielectric-porcelain composition is provided as one being capable of being fired at a temperature lower than a melting point of an Ag-based metal and being capable of demonstrating a sufficient bending strength even through firing at a low firing temperature. The dielectric-porcelain composition contains Mg2SiO4 as a major component and contains a zinc oxide, a boron oxide, an alkaline earth metal oxide, a copper compound, and a lithium compound as minor components.