Patent ID: 7714489

Claim:
A spark plug comprising: a center electrode extending in a direction of an axis of said spark plug; a cylindrical insulator which has a shaft hole, and which holds said center electrode in said shaft hole in a state where a tip end of said center electrode is projected from a tip end face of said insulator; a metal shell which holds said insulator; and a ground electrode in which one end is joined to a tip end face of said metal shell, and which extends from said one end toward another end, wherein said ground electrode has an inner side face which faces said center electrode, and an outer side face which is a back face with respect to said inner side face, said outer side face is formed as an arcuately curved face when viewed from a side of said another end in an extending direction of said ground electrode, and a columnar noble metal tip is joined to said inner side face and wherein, when said ground electrode is viewed from the side of said another end in the extending direction of said ground electrode, said ground electrode has: a flat face which is in said inner side face, and in which an outline of said ground electrode is formed as a linear portion; and two first arcuate portions which begin at end portions of said linear portion, and which are positioned on sides of ends of said linear portion, respectively, and said tip end face of said noble metal tip protrudes more than an intersection of two virtual extended arcs on a side of said inner side face, said virtual extended arcs having a radius curvature R which is equal to radius curvatures of said first arcuate portions, and being formed be by extending said first arcuate portions.