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1. A spark plug comprising:
a center electrode extending in a direction of an axis; a ceramic insulator having an axial bore extending in the direction of the axis and holding the center electrode in the axial bore; a metallic shell provided externally of an outer circumference of the ceramic insulator; and a ground electrode having an extending portion extending in the direction of the axis, attached to the metallic shell at one end, and forming, at the other end, a gap in cooperation with the center electrode, at least one of the center electrode and the ground electrode has an electrode base metal, a columnar noble metal tip disposed in such a manner as to face the other electrode, and an intermediate member disposed between the electrode base metal and the noble metal tip; the intermediate member has a first surface in contact with the noble metal tip and a second surface being in contact with the electrode base metal, located opposite the first surface, and having an area greater than that of a cross section of the noble metal tip cut by a plane parallel to the first surface; a nugget is formed through fusion and solidification resulting from welding at least a portion of a boundary between the intermediate member and the electrode base metal; and in a section of the spark plug cut by a plane which passes through the center of gravity of the intermediate member and is in parallel with a facing direction between the extending portion and the center electrode, a relational expression S1/(D1×H1)≧0.005 is satisfied, where S1 is a total area of the nugget, H1 is a height of an end surface of the noble metal tip from a disposition surface of the electrode base metal where the intermediate member is disposed, and D1 is a maximum width of the noble metal tip.
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