| 摘要 |
It is an object of the present invention to provide a spark plug that can be produced at low cost and secure compatibility between ignition performance and durability. The spark plug according to the present invention includes a center electrode and a ground electrode having an inner surface facing the center electrode, characterized in that; the ground electrode has a protrusion and a hole; the protrusion protrudes from the inner surface by a protruding length A of 0.4 to 1 mm and has a first straight portion with a projected first straight portion area S1 of 1.5 to 3 mm2; the hole has an opening at an outer surface of the ground electrode, a second straight portion, a bottom portion and a transition portion; the opening is formed in such a manner that the ratio (S2/S1) of a projected opening area S2 of the opening to the projected first straight portion area S1 is not less than 1.2; and the transition portion is in the form of a taper portion.
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