| 摘要 |
A spark plug that is equipped with an insulator that has a stem portion on a front end side, a center electrode, and a main metal member that retains the insulator at an engaging projection portion, wherein an inner diameter DIN of the engaging projection portion, the maximum outer diameter dOUT of the stem portion, which faces an inner circumferential surface of the engaging projection portion, its inner diameter dIN, and dielectric constant∈of the insulator satisfy the condition of (DIN−dOUT)/2≰0.40 (mm), (dOUT−dIN)/2≰1.65 (mm), and∈≧9.4.
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