Patent ID: 8564184

Claim:
A spark plug that is equipped with (a) an insulator that is generally cylindrically formed to have a small-diameter stem portion on a front end side and has a through hole passing though in an axial direction, (b) a center electrode inserted on the front end side of the through hole, and (c) a main metal member that is generally cylindrically formed to have an engaging projection portion projecting inwardly in the diametral direction and retains the insulator inserted thereinto, at the engaging projection portion, the spark plug being characterized by that, when an inner diameter of the engaging projection portion is represented by D IN (mm), the maximum outer diameter of a portion, which faces an inner circumferential surface of the engaging projection portion, of the stem portion is represented by d OUT (mm), and its inner diameter is represented by d IN (mm), and dielectric constant of the insulator is represented by ∈, condition (1) of “(D IN −d OUT )/2≦0.40 (mm)”, condition (2) of “(d OUT −d IN )/2≦1.65 (mm)”, and condition (3) of “∈≧9.4” are satisfied.