SPARK PLUG

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Disclosed is a spark plug which is provided with an insulating body that exhibits satisfactory withstand voltage characteristics and mechanical strength even in a high temperature environment higher than 700˚C. Specifically disclosed is a spark plug (1) which is provided with a center electrode (2), an insulating body (3) and a ground electrode (6). The spark plug (1) is characterized in that the insulating body (3) is configured of an alumina-based sintered body containing an Si content, a group 2 element (2A) content and a rare earth element (RE) content, and the alumina-based sintered body has an RE-β-alumina crystal phase. The spark plug (1) is also characterized in that the average crystal grain size of the RE-β-alumina crystal phase (DA(RE)) and the average crystal grain size of the alumina (DA(Al)) satisfy the following condition (1). Condition (1) : 0.2 ≤ DA(RE)/DA(Al) ≤ 3.0