Spark plug

A spark plug comprising:

• a center electrode including a rod portion extending in a direction of an axis and a head portion extending continuously from a rear end of the rod portion and having a diameter larger than a diameter of the rod portion;
• an insulator having an axial bore, the center electrode being disposed in the axial bore at a front end thereof;
• and
• a seal portion for fixing the center electrode in the axial bore, wherein
• the insulator has a leg portion inner circumferential surface which surrounds the rod portion, a cylindrical trunk portion inner circumferential surface which has a diameter larger than a diameter of the leg portion inner circumferential surface, surrounds the head portion, and has a center axis coinciding with the axis, and a support surface which supports the head portion and extends continuously between the leg portion inner circumferential surface and the trunk portion inner circumferential surface,
• the seal portion has a seal portion outer circumferential surface in contact with the trunk portion inner circumferential surface, a seal portion abutment surface in contact with the support surface, and a connecting surface extending continuously between the seal portion outer circumferential surface and the seal portion abutment surface, and,
• in a cross section of the seal portion cut along a plane including the axis, a curve, which is part of a contour of the seal portion appearing in the cross-section and is included in the connecting surface, has a radius of curvature R of 0.1 mm or more.