| 摘要 |
PROBLEM TO BE SOLVED: To reconcile ignitability and durability by maintaining the excellent ignitability even at discharge time to either earth electrode, and aerially discharging to the first earth electrode without blowing away a discharge spark by a gas flow in a spark plug having the first and second earth electrodes for ordinary time and carbon fouling time. SOLUTION: An insulating insulator 3 for holding the outer periphery of a central electrode 4 is supported in a cylindrical installing metal fitting 2 in a state of exposing a tip part. The the installing metal fitting 2 is provided with the first earth electrode 5 for forming a first discharge gap A between the electrode and a tip surface of the central electrode 4, and the second earth electrodes 6 and 7 for forming a second discharge gap B between the electrodes and a small diameter part 3c of the insulating insulator 3. When a shaft directional distance between a lower end surface of the insulating insulator 3 and a lower end surface of the installing metal fitting 2 is C, a shaft directional distance between the lower end surface of the insulating insulator 3 and the lower end edge of the second earth electrodes 6 and 7 is D, and a shaft directional distance between the lower edge of the second earth electrodes 6 and 7 and the lower end surface of the installing metal fitting 2 is E, the distances C, D and E are set so as to satisfy 1 mm<=C, 0 mm<=D, and 1 mm<=E.
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