SPARK PLUG

A spark plug with high dielectric strength is provided. The spark plug of the present invention includes a center electrode including a rod portion and a head portion extending continuously from the rear end of the rod portion and having a diameter larger than the diameter of the rod portion; an insulator having an axial bore, the center electrode being disposed in the axial bore at its front end; and a seal portion for fixing the center electrode in the axial bore. In this spark plug, 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 the diameter of the leg portion inner circumferential surface, surrounds the head portion, and has a center axis coinciding with the above 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. In a cross section of the seal portion cut along a plane including the axis, a curve which is part of the 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.