Patent ID: 8288929

Claim:
A spark plug comprising: a rod-shaped center electrode; a first noble metal tip joined to a front end of said center electrode; a substantially cylindrical insulator provided at an outer circumference of said center electrode; a tubular metallic shell provided at an outer circumference of said insulator; a ground electrode having a base end portion joined to a front end surface of said metallic shell, and a distal end portion located opposite to the base end portion and facing a front end portion of said center electrode, said ground electrode including an outer layer made of nickel alloy and an inner layer made of a material having a better thermal conductivity than the outer layer; and a second noble metal tip joined to the distal end portion of said ground electrode by way of a melted portion formed by one of laser welding and beam welding and forming a spark discharge gap between a tip end face of said second noble metal tip and a tip end face of the first noble metal tip, wherein, when a cross section of said ground electrode is taken along the axis of said second noble metal tip and viewed from the distal end side of said ground electrode: the protrusion height A of said second noble metal tip from said joining surface to the tip end face of said second noble metal tip is 0.4mm or more, said ground electrode includes a substantially flat joining surface to which said second noble metal tip is joined and an outwardly curved surface, said inner layer has at said joining surface side one of a substantially flat surface and recessed surface, and the minimum distance F between the said melted portion and said inner layer is 0.1mm or more, wherein assuming that in said cross section, W denotes the width of the tip end face of said second noble metal tip and C denotes the width of said inner layer, in the direction parallel to said joining surface, it is satisfied Wâ‰¦C.