SPARK PLUG

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The purpose is to provide a spark plug that exhibits minimal deterioration with respect to ambient temperature, even at high temperatures, as well as outstanding voltage withstand characteristics and mechanical strength. The spark plug (1) is provided with an insulator (3) fabricted from an alumina based sintered body containing an Al component, an Si component, and a Group 2 component and a rare earth component of the periodic table based on the IUPAC 1990 recommendations, at content percentages that satisfy conditions (1) and (2), and having a glass phase containing alumina particles and a crystalline phase satisfying condition (3). (1) 0.25 <= content percentage (mass%) of the Si component on an oxide conversion basis <= 1.25; (2) 1.35 <= [content percentage of the Si component on an oxide conversion basis /(content percentage of the Group 2 component on an oxide conversion basis/content percentage of the rare earth component on an oxide conversion basis)] <= 7.5; (3) contains at least one of a silicate crystalline phase and an aluminate crystalline phase of particle size within a range of 20-350 nm.