Insulator and spark plug

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An insulator used in a spark plug having improved strength and dielectric strength performance. The insulator comprises 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.