SPARK PLUG AND PROCESS FOR PRODUCING SPARK PLUG

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To provide a small-sized spark plug equipped with stain resistance and withstand voltage characteristics for a long term, and a process for producing this spark plug. A spark plug 100 that is equipped with an insulator 2 that has a stem portion 30 on a front end side, a center electrode 3, and a main metal member 1 that retains the insulator 2 at an engaging projection portion 56, wherein an inner diameter DIN of the engaging projection portion 56, the maximum outer diameter dOUT of the stem portion 30, which faces an inner circumferential surface 59 of the engaging projection portion 56, its inner diameter dIN, and dielectric constant ε of the insulator 2 satisfy the following condition. A process for producing the spark plug 100 containing a step of producing the insulator 2 by pressing and then sintering a raw material powder containing Al compound powder, Si compound powder, a powder, of a compound of an element of group 2, of at least two kinds containing Mg compound powder and Ba compound powder, and 0.5-4.0 mass % of a rare-earth compound powder. Condition : "(DIN - dOUT)/2 ≤ 0.40 (mm)", "(dOUT - dIN)/2 ≤ 1.65 (mm)", and "ε ≥ 9.4 (F/m)".