A dielectric ceramic obtained by firing a raw material comprising an alumina powder, a crystallizable glass powder mainly containing SiO.sub.2, CaO and MgO, and a noncrystallizable glass powder mainly containing SiO.sub.2, B.sub.2O.sub.3 and Na.sub.2O. After the firing, the dielectric ceramic contains an alumina crystal phase, a diopside crystal phase (Ca(Mg, Al)(Si, Al).sub.2O.sub.6) and a magnesia-spinel crystal phase (MgAl.sub.2O.sub.4) and has a porosity of not higher than 2.2% when measured by a mercury penetration method.