Patent ID: 8212463

Claim:
A spark plug comprising: a cylindrical insulating body having an axial hole extending in an axial direction; a center electrode inserted in the axial hole; a cylindrical metal shell provided on the outer periphery of the insulating body; a ground electrode disposed at the leading end portion of the metal shell; and a columnar noble metal tip of which one end face is joined to the leading end portion of the ground electrode, and the other end face forms a gap with the leading end portion of the center electrode, wherein the noble metal tip is joined to the ground electrode via a melt portion, wherein the noble metal tip and ground electrode melt together, formed by being irradiated with a laser beam or electron beam from a side surface side of the noble metal tip, and the melt portion, including an exposed surface which, being a region on the side irradiated with the laser beam or electron beam, is exposed on a surface of the ground electrode, is such that in a section including a central axis of the noble metal tip and perpendicular to a side surface of the ground electrode on the side irradiated with the laser beam or electron beam, ¾ or more of the one end face of the noble metal tip being joined to the ground electrode and, when the thickness of a thickest portion of the melt portion in a direction of the central axis of the noble metal tip is taken to be TX (mm), and the length of a longest portion of the melt portion in a direction of irradiation with the laser beam or electron beam is taken to be LX (mm), TX and LX satisfy 1.5≦LX/TX, and when a first straight line is drawn passing through a point AX, on the boundary between the melt portion and noble metal tip, positioned between the external line of one side surface of the noble metal tip positioned on the exposed surface side and the central axis of the noble metal tip, and a point BX thereon positioned between the external line of the other side surface of the noble metal tip and the central axis of the noble metal tip, a second straight line is drawn passing through the point BX and extending along the external line of the other end face of the noble metal tip, and an acute angle αX (°) among angles formed by the first straight line and second straight line is taken to be positive when formed on the noble metal tip side, and taken to be negative when formed on the ground electrode side, with the second straight line as a reference, the angle αX satisfies −15≦αX≦25.