SPARK PLUG

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A spark plug exhibits a satisfactory withstand voltage characteristic and sufficient mechanical strength in a high temperature environment exceeding 700° C. The spark plug has a center electrode, an insulator, and a ground electrode, characterized in that the insulator is formed of an alumina-based sintered material containing an Si component, a Group 2 element (2A) component, and a rare earth element (RE) component; that the alumina-based sintered material has an RE-β-alumina crystal phase; and that the mean crystal grain size DA(RE) of the RE-β-alumina crystal phase and that of alumina DA(Al) satisfy the following relationship (1) : [in-line-formulae]0.2≦DA(RE)/DA(Al)≦3.0.[/in-line-formulae]