A glow plug includes a ceramic heater having a resistance-heating element embedded at its end portion. In order to supply electricity to the resistance-heating element, first and second heater terminals are exposed on the surface of the ceramic heater. First and second terminal rings, which serve as metallic fitting members, are shrink-fitted to the surface of the ceramic heater in such a manner as to cover and be electrically connected to the exposed first and second heater terminals. A metal layer not higher than Ni in ionization tendency is formed on the inner circumferential surface of each of the first and second terminal rings.