Spark plug having particular insulator

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An object of the invention is to provide a spark plug including an insulator which is difficult to cause breakdown and exhibits higher withstand voltage characteristics. This invention relates to a spark plug 1 including a center electrode 2, an insulator 3 and a grounding electrode 6, wherein the insulator is formed by an alumina-based sintered body which has a theoretical density ratio of 95.0% or more, which contains a rare earth element component, Si component, and at least two kinds of Group 2 element (2A) components, and in which the rare earth element being dispersed so as to satisfy the following condition : when 7 places of measurement line (measurement length : 180 μm) are selected in the alumina-based sintered body and the rare earth element (RE) present in each of 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 places or more of the 7 places.