Patent ID: 6885137

Claim:
A spark plug comprising: a center electrode; an insulator holding said center electrode; a housing holding and fixing said insulator; and a ground electrode having one end portion connected to said housing and the other end portion opposed to said center electrode via a discharge gap intervening therebetween, wherein an Ir alloy firing tip made of a rod-like Ir alloy is connected to a portion of at least one of said center electrode and said ground electrode facing to said discharge gap, a cross-sectional area of said Ir alloy firing tip taken along a plane perpendicular to an axis of said Ir alloy firing tip has an out-of-round configuration, in said cross-sectional area of said Ir alloy firing tip, when it is assumed that a circumscribed circle has a largest diameter A among virtual circles each contacting at least three portions of a visible outline of said cross-sectional area and an inscribed circle has a largest diameter B among inscribed circles each being coaxial with said circumscribed circle, a ratio of said diameter B to said diameter A (i.e., B/A) is equal to or larger than 0.8 and is less than 1.0, the visible outline of said cross-sectional area is constituted by a serial joint of at least seven straight or curved line elements, and an angle between each line element and an adjacent line element is not less than 125° and is not larger than 235°.