A spark plug, comprising: an insulator having an axial hole extending in a direction of an axis; a center electrode inserted in a forward end portion of the axial hole; and a hollow cylindrical metal shell disposed on the outer periphery of the insulator, the insulator including an insulator nose length portion provided at the forward end of the insulator, and a tapered portion, extending from a rear end of the insulator nose length portion toward a rear end of the insulator in the axis direction, said tapered portion having a diameter increasing toward the axis direction rear end side, and the metal shell including a shoulder portion, protruding inward in a radial direction, having a retaining surface by which the tapered portion 11 is directly or indirectly retained, and a male thread portion, positioned on the outer peripheral side of the shoulder portion, for bringing the spark plug into threaded engagement with a mounting hole of a combustion device, wherein the thread diameter of the male thread portion is M12 or less, and when the area of a cross section of the insulator, perpendicular to the axis, passing through a boundary between the insulator nose length portion and the 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, wherein the insulator includes a middle barrel portion, extending from the rear end of the tapered portion toward the rear end of the insulator in the axis direction, larger in diameter than the insulator nose length portion, and an increased diameter portion, extending from the rear end of the middle barrel portion toward the rear end of the insulator in the axis direction, the outside diameter of which is increased toward the axis direction rear end side, characterized in that when the area of a cross section of the insulator, perpendicular to the axis, passing through a boundary between the middle barrel portion and increased diameter portion is A (mm 2), and the mass of the spark plug is M (g), M/A ≤ 1.40 (g/mm 2) is satisfied. an insulator having an axial hole extending in a direction of an axis; a center electrode inserted in a forward end portion of the axial hole; and a hollow cylindrical metal shell disposed on the outer periphery of the insulator, the insulator including an insulator nose length portion provided at the forward end of the insulator, and a tapered portion, extending from a rear end of the insulator nose length portion toward a rear end of the insulator in the axis direction, said tapered portion having a diameter increasing toward the axis direction rear end side, and the metal shell including a shoulder portion, protruding inward in a radial direction, having a retaining surface by which the tapered portion 11 is directly or indirectly retained, and a male thread portion, positioned on the outer peripheral side of the shoulder portion, for bringing the spark plug into threaded engagement with a mounting hole of a combustion device, wherein the thread diameter of the male thread portion is M12 or less, and when the area of a cross section of the insulator, perpendicular to the axis, passing through a boundary between the insulator nose length portion and the 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, wherein the insulator includes a middle barrel portion, extending from the rear end of the tapered portion toward the rear end of the insulator in the axis direction, larger in diameter than the insulator nose length portion, and an increased diameter portion, extending from the rear end of the middle barrel portion toward the rear end of the insulator in the axis direction, the outside diameter of which is increased toward the axis direction rear end side, characterized in that when the area of a cross section of the insulator, perpendicular to the axis, passing through a boundary between the middle barrel portion and increased diameter portion is A (mm 2), and the mass of the spark plug is M (g), M/A ≤ 1.40 (g/mm 2) is satisfied.