Patent ID: 8531095

Claim:
A spark plug comprising: a rod shape center electrode; a cylindrical shape insulating body that has an axial hole extending in an axis direction and includes the center electrode at a leading end side of the axial hole; and a cylindrical shape metal shell that surrounds and holds a periphery of the insulating body in a state that a leading end section of the insulating body is extended further than its leading end surface, wherein the insulating body includes: a foot section that is positioned at the leading end section; a tapered section that is extended toward a rear end side from a rear end of the foot section and has a diameter that is widened toward the rear end side; and a middle body section that is extended toward the rear end side from a rear end of the tapered section and has a diameter that is larger than the foot section, wherein a step section and a leading end side inner periphery section that is positioned at a leading end side of the step section are formed in the inner periphery of the metal shell, wherein the insulating body is fixed at the metal shell in a state that the tapered section is directly or indirectly engaged with respect to the step section, wherein a maximum outer diameter of a portion, which is arranged within the foot section, of the center electrode is 3.0 mm or less, wherein assuming that A (mm 3 ) is a volume of a portion of 2 mm of the insulating body from a leading end of the insulating body toward a rear end side along the axis direction and B (mm 3 ) is a volume of a portion of the insulating body from a rear end of the portion of the tapered section which is engaged to the step section to a leading end side and in which a diameter difference between the leading end side inner periphery section and its outer periphery portion is 1.5 mm or less, 0.12≦A/B≦0.24 is satisfied.