The present invention addresses the problem of providing a spark plug with a superior voltage withstanding performance. The spark plug according to the present invention is provided with: a center electrode that has a rod-like section and a head section, the head section being continuously connected to the posterior side of the rod-like section and having a larger diameter than the diameter of the rod-like section; an insulator in which the center electrode is positioned on the anterior side inside a shaft hole; and a seal section for fixing the center electrode inside the shaft hole. The insulator has: a leg-section inner circumferential surface that surrounds the rod-like section; a cylindrical trunk-section inner circumferential surface that has a diameter larger than the diameter of the leg-section inner circumferential surface, surrounds the head section, and has a central axis in alignment with the axis line; and a supporting surface that supports the head section and is continuously formed between the leg-section inner circumferential surface and the trunk-section inner circumferential surface. The seal section has: a seal-section outer circumferential surface that is in contact with the trunk-section inner circumferential surface; a seal-section receiving surface that is in contact with the supporting surface; and a continuous surface that is continuously formed between the seal-section outer circumferential surface and the seal-section receiving surface. When the seal section is cut along a plane that includes the axis line, among the outlines that appear in the cross-section of the seal section, a curve included in the continuous surface has a curvature radius (R) equal to or greater than 0.1 mm.