Patent ID: 7906894

Claim:
A spark plug for an internal combustion engine, comprising: a cylindrical metal shell arranged in an axial direction of the spark plug; a cylindrical insulator retained in the metal shell; a column-shaped center electrode disposed in the insulator with a front end thereof protruding from the insulator; and a ground electrode joined at a rear end thereof to a front end of the metal shell and bent in such a manner that a front end of the ground electrode extends toward an axis of the spark plug so as to define a spark gap between the front end of the center electrode and the front end of the ground electrode, the ground electrode including a flat region formed on an outer peripheral surface thereof opposite to an inner peripheral surface facing the insulator, the flat region being located on the front end of the ground electrode and having a length of 0.2 mm or more from a front end face of the ground electrode in a longitudinal direction of the ground electrode, any region other than the flat region of the outer peripheral surface of the ground electrode being convex curved, the ground electrode satisfying the following dimensional condition (1) with respect to a first cross section of the ground electrode taken through said any region other than the flat region in a direction perpendicular to the longitudinal direction of the ground electrode and a second cross section of the ground electrode taken through the flat region in a direction perpendicular to the longitudinal direction of the ground electrode, 0.950≦( S 2/ L 2)/( S 1/ L 1)≦0.995 (1) where S 1 is the area of the first cross section; L 1 is the perimeter of the first cross section; S 2 is the area of the second cross section; and L 2 is the perimeter of the second cross section.