Patent ID: 8749125

Claim:
A spark plug equipped with a center electrode, a substantially cylindrical insulator provided on a periphery of the center electrode, and a substantially cylindrical metal shell provided on a periphery of the insulator, the spark plug being characterized by that the insulator is equipped with an alumina-based sintered body containing silicon component (hereinafter referred to as Si component), magnesium component (hereinafter referred to as Mg component), at least one component (hereinafter referred to as group 2 element component) selected from calcium component, strontium component, and barium component, and a rare earth element component (hereinafter referred to as RE component), that a grain boundary phase of the alumina-based sintered body has a hexaaluminate crystal containing the RE component, that, in the alumina-based sintered body, ratio of the RE component to the Mg component by mass ratio in terms of oxide (RE oxide/MgO) satisfies 5.4≦RE oxide/MgO≦17.5, and that content of an alkali metal component in terms of oxide in a spot, where the hexaaluminate crystal is present, of circular spots having a diameter of 0.3 nm ranges from 0.01 mass % to 20 mass % when the grain boundary phase of the alumina-based sintered body is observed with a transmission electron microscope, wherein the spot where the alkali metal component content is in the range contains the hexaaluminate crystal and the Si component.