This invention provides a ceramic heater comprising a core, an insulation layer and a resistance heating element of high-melting metal as embedded between the core and insulation layer, wherein the operating temperature is not less than 300° C., the insulation layer comprising a sintered compact composed of 88 to 95 weight % of Al2O3 supplemented with, as sintering aids, 3 to 10 weight % of SiO2, 0.4 to 1.0 weight % of MgO and 1.0 to 2.5 weight % of CaO and having a density of not less than 3.60 and a thickness of 100 to 300 &mgr;m.