A spark plug includes an insulator having an axial hole, a center electrode disposed in the axial hole and having a forward end portion projecting to a forward end of the axial hole, a tubular metallic shell which holds the insulator on its inner circumference and has a screw portion formed on its outer circumferential surface, and a ground electrode whose first end portion is inserted into a through hole provided in the metallic shell and is welded to the through hole and whose second end portion forms a discharge gap between the second end portion and the forward end portion. The screw portion has a first screw portion located on a rear end of the through hole and a second screw portion located on a forward end of the through hole, and the second screw portion has a pitch diameter larger than that of the first screw portion.