A spark plug for an internal combustion engine provided with a cylindrical housing, a ceramic insulator, a center electrode and a ground electrode. The ground electrode has a body base disposed from a front end surface of the cylindrical housing to a front end-side thereof. The ground electrode has a spark discharge gap formed therebetween itself and the center electrode. The body base is provided with pair of side-connecting surfaces which connect the inner surface and the outer surface. Each of the side-connecting surfaces having a side flat surface which is a flat surface parallel to an aligning direction of the center electrode and the body base. A distance between the pair of the side flat surfaces is a maximum width of the body base. A minimum distance between the inner surface and the side flat surface satisfies a relationship of 0.1 mm≦L≦0.5 mm, in the aligning direction.