A spark plug includes a metallic shell which holds a center electrode in an insulated condition, and a ground electrode is held in a penetration hole of the metallic shell. The penetration hole includes a circular counterbore portion, and a penetrating portion extending from the counterbore portion to an inner circumferential surface of the metallic shell. The ground electrode includes a fixing portion which is fixed to the counterbore portion, and an extension portion extending from the fixing portion. The extension portion has a flat surface which faces a forward end surface of the center electrode in the axial direction. The penetrating portion restricts the extension portion such that the flat surface of the extension portion faces toward a rear end side in the axial direction.