SPARK PLUG

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To provide a spark plug having an insulator which exhibits a satisfactory withstand voltage characteristic and sufficient mechanical strength in a high temperature environment exceeding 700°C. The invention provides a spark plug 1 having a center electrode 2, an insulator 3, and a ground electrode 6, characterized in that the insulator 3 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-²-alumina crystal phase; and that the mean crystal grain size D A (RE) of the RE-²-alumina crystal phase and that of alumina D A (Al) satisfy the following relationship (1) : 0.2‰¤D A (RE)/D A (Al)‰¤3.0.