Patent ID: 8704434

Claim:
A spark plug, comprising: a cylindrical insulator extending in a direction of an axis; and a cylindrical metal shell provided on the outer periphery of the insulator, the metal shell having: a thread portion for threadedly engaging with a mounting hole of a combustion device; and a flanged seat portion, having a tapered surface whose outer diameter decreases gradually toward a leading end side, which is positioned closer to a rear end side than the thread portion, wherein the tapered surface coming into contact with a bearing surface of the combustion device when the thread portion is threadedly engaged with the mounting hole of the combustion device, annular protruding portions, or a helical protruding portion having the length of one or more turns, extending in a circumferential direction of the metal shell, is formed within at least a range on the tapered surface from an outermost peripheral portion to a part forming an outer diameter occupying 95% of the outer diameter of the outermost peripheral portion, and the arithmetic mean roughness of a surface of the tapered surface within the range is set to 1 μm or more and 5 μm or less on a section including the axis.