Spark plug

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PROBLEM TO BE SOLVED : To provide a spark plug having an insulator which exhibits sufficient voltage withstanding characteristics and mechanical strength even in a high temperature environment exceeding, for example, 700°C. SOLUTION : The spark plug 1 includes a center electrode 2, an insulator 3, and a ground electrode 6. The insulator 3 is constructed of an alumina group sintered compact containing a Si component, a Group 2 element (2A), and a rare earth element (RE), and this alumina group sintered compact has an RE-β-alumina crystal phase, and the average crystal particle size DA of the RE-β-alumina crystal phase and the average crystal particle size DA(Al) of alumina satisfies the following condition (1). Condition (1) : 0.2≤DA(RE)/DA(Al)≤3.0. COPYRIGHT : (C)2011, JPO&INPIT