Spark Plug

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A spark plug comprising : a center electrode; a metal shell; and an alumina ceramic insulator disposed between the center electrode and the metal shell, wherein at least part of the surface of the insulator is covered with a glaze layer comprising oxides, the glaze layer comprising : 1 mol% or less of a Pb component in terms of PbO; 40 to 60 mol% of a si component in terms of SiO2; 20 to 40 mol% of a B component in terms of B2O3; 0.5 to 25 mol% of a Zn component in terms of ZnO; 0.5 to 15 mol% in total of at least one of Ba and Sr components in terms of BaO and SrO, respectively; 2 to 12 mol% in total of at least one alkaline metal component of Na, K and Li, in terms of Na2O, K2O, and Li2O, respectively, wherein K is essential; and 0.1 to 5 mol% in total of at least one component of Bi, Sb and rare earth RE, RE being at least one selected from Sc, Y, La, Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb and Lu, in terms of Bi2O3, Sb2O5 and RE2O3, respectively, proviso that Ce is in terms of CeO2 and Pr is in terms of Pr7O11, wherein the glaze layer comprises 8 to 30 mol% in total of the Zn component and the at least one of Ba and Sr components in terms of ZnO, BaO and SrO, respectively.