SPARK PLUG AND PROCESS FOR PRODUCING SPARK PLUG

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Provided are a small spark plug which combines resistance to fouling and chipping with long-term withstand voltage characteristics, and a process for producing the spark plug. The spark plug (100) comprises an insulator (2) having a sleeve part (30) on the tip side, a center electrode (3), and a main metal fitting (1) that holds the insulator (2) by means of a projecting part for engagement (56), wherein the inner diameter (DIN) of the projecting part for engagement (56), the maximum outer diameter (dOUT) of the part of the sleeve part (30) which faces the inner peripheral surface (59) of the projecting part for engagement (56), the inner diameter (dIN) of said part of the sleeve part (30), and the permittivity (ε) of the insulator (2) satisfy the requirements below. The process for producing the spark plug (100) includes a step in which raw-material powders comprising an Al compound powder, an Si compound powder, at least two Group-2 element compound powders comprising an Mg compound powder and a Ba compound powder, and 0.5-4.0 mass% rare-earth compound powder is press-molded and then sintered to produce the insulator (2). Requirements : (DIN-dOUT)/2≤0.40 (mm), (dOUT-dIN)/2≤1.65 (mm), and ε≥9.4