Patent ID: 7164225

Claim:
A spark plug comprising: an insulator having an axial hole in an axial direction of the spark plug; a center electrode disposed in a tip end side of the axial hole of said insulator so as to project a tip end of said center electrode from said tip end side of the insulator, said center electrode including: an electrode base member made of pure Ni or an Ni alloy containing 85 wt% or more of Ni; and a noble metal chip fixed to a tip end of said electrode base member; a metal shell surrounding said insulator; and a ground electrode in which one end is joined to said metal shell, and another end portion opposes said noble metal chip to form a spark discharge gap between said ground electrode and said noble metal chip, wherein when an outer diameter of a tip end of said metal shell is M, an inner diameter of said tip end of said metal shell is D 1 , and an outer diameter of said center electrode in a virtual plane containing a tip end of said insulator is D 2 , the following relationships are satisfied: M≦10.1 mm, 0.5 mm≦D2<1.4 mm, and D1/D2≧3.5.