SPARK PLUG

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Provided is a spark plug having excellent 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 (Si component), a magnesium component (Mg component), at least one of a calcium component, a strontium component, and a barium component, and a rare-earth element component (RE component), that a grain boundary phase of the alumina-based sintered object has hexaaluminate crystals containing the RE component, that in the alumina-based sintered object, the weight ratio of the RE component to the Mg component satisfies 5.4≤(RE oxide)/MgO≤17.5, and that in circular spots having a diameter of 0.3 nm which are located in the grain boundary phase and in which the presence of the hexaaluminate crystals or the presence of both the hexaaluminate crystals and the Si component has been ascertained with a TEM, the content of an alkali metal component is 0.01-20 wt.% in terms of oxide amount.