In an outer electrode, a rear end of which is securely connected to a metallic shell of a spark plug by means of welding to form a spark gap between a front end of the outer electrode and a firing tip of a center electrode which is concentrically placed within the metallic shell through an insulator, a middle core which is provided which is made of copper, and the middle core is clad by a heat and spark-erosion resistant metal. A centermost core is clad by the middle core. The centermost core is made of a similar metal to the metallic shell, and rear end of the centermost core is welded to the metallic shell so as to reinforce a welding portion between the metallic shell and the outer electrode.