A spark plug is attached to an internal combustion engine (engine) or the like, and is used to ignite, for example, a fuel-air mixture within a combustion chamber. In general, such a spark plug includes an insulator having an axial hole extending in the axial direction; a center electrode inserted into a forward end portion of the axial hole; a metallic shell provided around the insulator; and a ground electrode fixed to a forward end portion of the metallic shell. A high voltage is applied to a gap formed between a distal end portion of the ground electrode and a forward end portion of the center electrode. As a result, spark discharge occurs, whereby the fuel-air mixture or the like is ignited.
Incidentally, when the size of the above-mentioned gap increases as a result of corrosion of the center electrode and the ground electrode caused by the spark discharge or the like, the voltage (discharge voltage) necessary for generating spark discharge increases.
When the discharge voltage becomes excessively large, generation of spark discharge becomes impossible (so-called misfire occurs).
One conceivable method of preventing a rapid increase in the gap to thereby extend the service life of the spark plug is providing an annular portion at a forward end portion of the ground electrode and forming the above-mentioned gap between an inner circumferential surface of the annular portion and an outer circumferential surface of the center electrode. Since this method realizes uniform corrosion of the entire circumference of the center electrode, a rapid increase in the above-mentioned gap can be prevented effectively. Also, in recent years, there has been proposed a technique of further extending the service life of the spark plug. According to the proposed technique, an annular tip formed of a metal (e.g., metal including iridium, platinum, or the like) which is excellent in corrosion resistance is joined to a part of the inner circumference of the annular portion, which part forms the above-mentioned gap in cooperation with the outer circumferential surface of the center electrode (see, for example, Patent Documents 1 and 2).