A low-temperature firable ceramic substrate having formed in its inside a ferrite layer with a coil embedded therein is provided. The ferrite layer has a composition of 63 to 73% by mass of Fe.sub.2O.sub.3, 5 to 10% by mass of CuO, 5 to 12% by mass of NiO, and 10 to 23% by mass of ZnO. Even if firing is performed at a lower temperature than ever, it is possible to obtain a ferrite layer which is excellent in sintered density and magnetic permeability.