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 D IN of the engaging projection portion 56, the maximum outer diameter d OUT 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 µ 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: "(D IN - d OUT )/2 ‰¤ 0.40 (mm)", "(d OUT - d IN )/2 ‰¤ 1.65 (mm)", and "µ ‰¥ 9.4 (F/m)".