PURPOSE: To prevent sticking of carbon due to heating of a heater, and improve withstand voltage performance by covering/fixing the heater arranged in a recessed part of an insulator leg part with an alumina coating film and high softening point glass.

CONSTITUTION: A recessed part 11 is arranged in an insulator leg part 10 of an insulator 2 to hold a center electrode 3 on the tip of a shaft hole. A heater 13 formed by baking metallic paste connected to a lead wire buried along a current-carrying ring above the insulator and a surface in the axial direction of the insulator 2, is arranged in this recessed part 11. This heater 13 is covered with/fixed by high softening point glass 15 to sandwich an alumina coating film 14 between it, and a thickness of the glass 15 is set in 30-500μm. Thereby, withstand voltage performance is improved, and sticking of carbon to the insulator leg part due to heating of the heater is prevented.