SPARK PLUG

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A task of this invention is to provide a spark plug that is superior in withstand voltage characteristics at high temperatures. A spark plug of this invention is equipped with a center electrode, an insulator, and a metal shell, and is characterized by that the insulator is equipped with an alumina-based sintered body containing silicon component (Si component), magnesium component (Mg component), at least one component of calcium component, strontium component, and barium component, and a rare earth element component (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, the weight ratio of the RE component to the Mg component is 5.4 ≤ RE oxide/MgO ≤ 17.5, and that the content of an alkali metal component in terms of oxide in a spot having a diameter of 0.3nm, where the hexaaluminate crystal or the hexaaluminate crystal and Si component in the grain boundary phase is present by TEM, ranges from 0.01 weight % to 20 weight %.