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1. A spark plug for internal-combustion engines comprising:
a mount fitting having a perimeter provided with a screw part for attachment formed on; a porcelain insulator that is held inside the mount fitting and that has an insulator tip part arranged to be projected outwardly from the mount fitting; a center electrode that is held inside the porcelain insulator and that has an electrode tip part arranged to be projected outwardly from the porcelain insulator; and a ground electrode where a spark discharge gap is formed between the ground electrode and the center electrode while a precious metal is provided on an electric discharge surface being opposed to the center electrode, the electric discharge surface being part of an opposing surface of the ground electrode, the opposing surface being opposed to the center electrode, wherein, the ground electrode is fixed to the mount fitting, while the ground electrode has a convex portion; the convex portion is made of a part of base materials of the ground electrode, and is projected toward the center electrode from the opposing surface of the ground electrode; the convex portion has a fusion solidification part made of the precious metal and the part of the base materials and is formed on an entire area of the electric discharge surface to be solidified at the entire electric discharge surface, wherein the fusion solidification part is produced by melting and solidifying the precious metal and the part of the base materials, the ground electrode has a concave portion formed on a rear side of the ground electrode, the concave portion being formed toward the opposing surface, the rear side being back-to-back to the opposing surface; and the convex portion is formed such that an extension of an axis of the convex portion passes through an area in which the concave portion is formed on the rear side, wherein, when an area of an opening of the concave portion is set to S1, and an average cross-section area of a section of the convex portion, which section intersects perpendicularly to an axial direction of the spark plug, is set to s, the relation of S1>=s is satisfied; and when an average cross-section area of a section of the concave portion, which section intersects perpendicularly in the axial direction, is set to S2, the relation of S2>=s is satisfied.
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