SPARK PLUG

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A spark plug includes an insulator made of an alumina sintered body containing Al2O3 as a principal component and additional components including; an Si component, a Ba component, an Mg component, a Ca component, an Sr component, and a rare earth element component. When the additional components are expressed as oxides including RSiO2, RBaO, RMgO, RCaO, RSrO, and RRE2O3, contents (mass %) of the sub-component satisfy expressions (1) to (6) as follows : (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.