Patent ID: 8215277

Claim:
A spark plug comprising: a center electrode extending in an axial direction; an insulator which has an axial hole extending in the axial direction and holds the center electrode on a front end side of an interior of the axial hole; a metal shell for holding the insulator by surrounding its periphery in a subassembly in which the center electrode is held in the axial hole of the insulator; and a ground electrode comprising one end portion joined to the metal shell and another end portion, a spark discharge gap being formed between the another end portion and the center electrode, wherein the following formula is satisfied: H≧ 1.8 mm, and the following formulae are satisfied: 4.02 mm 3 <Vi≦ 12.51 mm 3 ; 2.10 mm 3 ≦Vc≦ 6.42 mm 3 ; and Vc/Vi≦ 1.03, where: H is a length of the insulator protruding from a front end face of the metal shell toward a front end side thereof in the axial direction; Vi is a volume of a portion of the insulator which corresponds to a range of 1.5 mm from a front end of the insulator toward a rear end thereof in the axial direction; and Vc is a volume of a portion of the center electrode which corresponds to the range of 1.5 mm in the axial direction.