SPARK PLUG AND PROCESS FOR PRODUCING SPARK PLUG

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Provided is a spark plug which has high withstand voltage characteristics at high temperatures and is less apt to decrease in withstand voltage characteristics at high temperatures. The spark plug is equipped with a center electrode, an insulator, and a metal shell, and is characterized in that the insulator comprises an alumina-based sintered object containing a silicon component, a Group-2- element component, and a rare-earth element component, that the alumina-based sintered object comprises a glass phase and an alumina crystal phase, the two phases being determined by X-ray diffractometry, and that when a grain boundary phase of the alumina-based sintered object is examined with a transition electron microscope and the weight ratio of an alkali metal component to the rare-earth element component in terms of oxide amount [alkali metal/rare-earth element] in each of 30 circular spots having a diameter of 0.3 nm is calculated, then the average of the values thereof is 0.01-1.0.