This invention addresses the problem of providing a spark plug (1) formed by securely welding a ground electrode (6) by electrical resistance welding. The spark plug (1) is formed by providing a cylindrical main body metal fitting (4) and a ground electrode (6) electrical resistance-welded to the end of the main body metal fitting (4). The spark plug (1) is characterized in that the main body metal fitting (4) has a high-hardness region (42) with a Vickers hardness of 3×102 - 5×102 Hv, and in the cross section of the high-hardness region (42) in the plane containing the axis line of the spark plug (1) and the axis line of the ground electrode (6), the length d from the weld interface on the ground electrode (6) side to the weld interface on the main body metal fitting (4) side in the load direction at the time of the electrical resistance welding is between 0.3 mm and 0.8 mm.
