Patent ID: 7187110

Claim:
A spark plug comprising: an insulator having an axial hole formed in an axial direction along an axis of said spark plug; a center electrode disposed in said axial hole of said insulator and on a leading end side of said axial hole; a metal shell surrounding said insulator; and a ground electrode including a ground electrode body having one end joined to said metal shell, and a precious metal columnar tip having a constant diameter bonded to the other end portion of said ground electrode body through a joint portion and disposed opposite said center electrode to form a spark discharge gap between said center electrode and said ground electrode, wherein said spark plug satisfies the relations: t≧0.3 mm, and St/Sw≦ 7 in which t is an axial-direction shortest distance between a leading end surface of said precious metal columnar tip and said joint portion, St is a surface area of said precious metal columnar tip and said joint portion, and Sw is an area of said precious metal columnar tip and said joint portion as viewed from an inner circumferential surface of said ground electrode body, wherein said joint portion is formed by laser welding of said precious metal tip and said ground electrode body; and a radial length of La of said joint portion on one end side of said ground electrode is larger than a radial length of Lb of said joint portion on another end side of said ground electrode as viewed from said inner circumferential surface of said ground electrode body.