SPARK PLUG

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To provide a spark plug exhibiting excellent withstand voltage characteristics and mechanical strength at high temperature, which are comparable to those at ambient temperature. The spark plug 1 has an insulator 3 formed of an alumina-based sintered material, wherein the alumina-based sintered material contains an Al component, an Si component, a Group 2 element component, the Group included in the periodic table defined by Recommendations 1990, IUPAC, and a rare earth element component in such amounts that satisfy a condition (1) : 0.25 ≤ oxide-reduced amount (mass%) of the Si component ≤ 1.25, and a condition (2) : 1.35 ≤ [(oxide-reduced amount of the Si component)/((oxide-reduced amount of the Group 2 element component)/(oxide-reduced amount of the rare earth element component))] ≤ 7.5; and the alumina-based sintered material includes alumina grains, and a glass phase including a crystal phase that satisfies a condition (3) that the crystal phase includes at least one of an aluminate crystal phase and a silicate crystal phase, each having a grain size of 20 to 350 nm.