A spark plug comprising: a center electrode ; a substantially cylindrical insulator provided on a periphery of the center electrode ; and a grounding electrode provided such that one end thereof faces the center electrode through a spark discharge gap (G), wherein the insulator is formed by an alumina-based sintered body which contains a rare earth element (RE) component, wherein the rare earth element (RE) component is at least one component selected from a group consisting of La component, Pr component, Nd component and Sm component, and, an Si component, and a Group 2 element component containing Mg and Ba and further containing at least one other element excluding Mg and Ba in Group 2 elements in a periodic table based on a recommendation of IUPAC 1990, characterized in that the alumina-based sintered body has a theoretical density ratio of 95.0 % or more wherein the alumina-based sintered body has RE-β-alumina crystal phase containing at least the rare earth element (RE) component, and an average crystal particle diameter D A (RE) of the RE-β-alumina crystal phase and an average crystal particle diameter D A (Al) of alumina are satisfied with the following condition : Condition : 0.2 ≤ D A (RE) / D A (Al) ≤ 3.0, and in which the rare earth element (RE) is dispersed so as to satisfy the condition described below: when 7 measurement lines, each having a measurement length of 180 μm, are selected in the alumina-based sintered body and the rare earth element (RE) present in each of the selected measurement lines is analyzed with an energy dispersion method, a total number of measurement lines in which 8 or more peaks derived from the rare earth element (RE) are observed is 4 lines or more of the 7 lines. a center electrode ; a substantially cylindrical insulator provided on a periphery of the center electrode ; and a grounding electrode provided such that one end thereof faces the center electrode through a spark discharge gap (G), wherein the insulator is formed by an alumina-based sintered body which contains a rare earth element (RE) component, wherein the rare earth element (RE) component is at least one component selected from a group consisting of La component, Pr component, Nd component and Sm component, and, an Si component, and a Group 2 element component containing Mg and Ba and further containing at least one other element excluding Mg and Ba in Group 2 elements in a periodic table based on a recommendation of IUPAC 1990, characterized in that the alumina-based sintered body has a theoretical density ratio of 95.0 % or more wherein the alumina-based sintered body has RE-β-alumina crystal phase containing at least the rare earth element (RE) component, and an average crystal particle diameter D A (RE) of the RE-β-alumina crystal phase and an average crystal particle diameter D A (Al) of alumina are satisfied with the following condition : Condition : 0.2 ≤ D A (RE) / D A (Al) ≤ 3.0, and in which the rare earth element (RE) is dispersed so as to satisfy the condition described below: when 7 measurement lines, each having a measurement length of 180 μm, are selected in the alumina-based sintered body and the rare earth element (RE) present in each of the selected measurement lines is analyzed with an energy dispersion method, a total number of measurement lines in which 8 or more peaks derived from the rare earth element (RE) are observed is 4 lines or more of the 7 lines. <EOS>