Spark plug

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PROBLEM TO BE SOLVED : To provide a spark plug with an insulator which can keep a voltage-resistance performance under a high-temperature condition over a long period of time. SOLUTION : A spark plug comprises an insulator composed of an alumina sintered body. The insulator includes Al2O3 as a primary component. Supposing that in the insulator, the contents (mass%) of Si, Ba, Mg, Ca, Sr and rare earth element components in terms of oxide are RSiO2, RBaO, RMgO, RCaO, RSrO and RRE2O3 respectively, the contents of the Si, Ba, Mg, Ca, Sr and rare earth element components satisfy the following expressions (1)-(6) : (1) 1.0≤RSiO2≤5.0; (2) 0.5≤RBaO≤5.0; (3) 0≤RMgO≤0.18; (4) 0≤RMgO/RBaO≤0.36; (5) 0.3≤(RMgO+RCaO+RSrO)≤1.8; and (6) 0≤RRE2O3≤0.1. SELECTED DRAWING : Figure 1 COPYRIGHT : (C)2017, JPO&INPIT