[Object] To prevent a sealant from intruding between a flange and a stepped portion during manufacture of a spark plug and to prevent cracking of an insulator during use of the spark plug.
[Solution] A spark plug includes a center electrode including a flange, an insulator having a through-hole extending in an axial-line direction and holding the center electrode, and a sealant filled in the through-hole to fix the flange and the insulator to each other. The insulator includes a stepped portion in which the through-hole reduces a diameter toward a front end side and that supports the flange, and a small-diameter portion that is continuous with a front end side of the stepped portion and in which the diameter of the through-hole is smaller than in the stepped portion. An angle θ1 formed by the stepped portion and a plane perpendicular to the axial line and an angle θ2 formed by the plane perpendicular to the axial line and an opposing surface of the flange opposing the stepped portion satisfies θ1−θ2≥6°. In a cross section including the axial line, a maximum diameter D1 of the flange and a diameter D2 of the through-hole at a rear end of the small-diameter portion in the axial-line direction satisfy 0.15 mm≤(D1−D2)/2.