A spark plug includes: a center electrode including a rod portion and a head 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. The insulator comprises: a leg portion inner circumferential surface; a cylindrical trunk portion inner circumferential surface which surrounds the head portion; and a support surface which supports the head portion. The seal portion has: a seal portion outer circumferential surface; a seal portion abutment surface; and a connecting surface. In a cross section of the seal portion, a curve, which is part of the contour of the seal portion and is included in the connecting surface, has a radius of curvature R of 0.1 mm or more.