US Patent 7944134 Spark plug with center electrode having high heat dissipation property

A spark plug including: a center electrode including an electrode base member and a core member; an electrode tip joined to the center electrode via a molten bond; an insulator holding the center electrode; a metal shell holding the insulator; and a ground electrode joined to the metal shell. Relationships d≦2.1 [mm] and −0.09×d+0.33<Vb/(Va+Vb)<−0.2×d+0.75 are satisfied, where d is an outer diameter of the center electrode at a position 4 mm closer to the base end of the center electrode from the base end of the molten bond in the axial direction, and Va and Vb are volumes of the electrode base member and the core member, respectively, at a region between the base end of the molten bond and the position in the axial direction.