Spark plug

With a spark plug reduced in diameter, a crack of an insulator in the boundary between an insulator nose length portion and a tapered portion is more reliably prevented, thus realizing a superior durability. A spark plug includes an insulator and a metal shell. The insulator includes an insulator nose length portion positioned in a leading end portion, and a tapered portion, extending from the rear end of the insulator nose length portion toward a rear end side, increased in diameter toward the rear end side. The metal shell includes a shoulder portion, protruding inward in a radial direction, having a retaining surface by which the tapered portion is retained, and a male thread portion positioned on the outer peripheral side of the shoulder portion, and the thread diameter of the male thread portion is set to M12 or less. When the area of a cross section of the insulator, perpendicular to an axis, passing through the boundary between the insulator nose length section and tapered portion is B (mm 2 ), and the area of a cross section of the metal shell, perpendicular to the axis, passing through the leading end of the retaining surface is C (mm 2 ), 2.80 ‰¤C/B ‰¤3.50 is satisfied.