Insulator and spark plug

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[Objective] To improve strength and dielectric strength performance of an insulator used in a spark plug [Means for Solution] In an insulator used in a spark plug and comprising a sintered compact whose main constituent is alumina, in the sintered compact, a content ratio of a rare earth element (R. E.) and a group 2 element (2A) satisfies 0.1 ≤ R. E./2A ≤ 1.4, and the content ratio of the R. E. and barium oxide (BaO) satisfies 0.2 ≤ BaO/R. E. ≤ 0.8. At least one virtual rectangular frame of 7.5 µm × 50 µm surrounding crystals containing the rare earth element lies within a region of 630 µm × 480 µm in any cross section of the sintered compact. An occupancy rate of an area of the crystal containing the rare earth element to an area of the rectangular frame is greater than or equal to 5% and, in each of the divided regions when the rectangular frame is equally divided into three in a direction of a long side, a ratio of the maximum area occupancy rate and the minimum area occupancy rate of the area occupancy rates of the crystal containing the rare earth element is less than or equal to 5.5%.