Patent ID: 7368864

Claim:
A spark plug comprising: an insulator having an axial hole in an axial direction of said spark plug; a center electrode disposed in a tip end side of said axial hole of said insulator; a metal shell surrounding said insulator; a first ground electrode including a first ground electrode body having one end bonded to said metal shell; and a noble metal tip joined to an inner side face of another end portion of said first ground electrode body on a side of said center electrode and disposed opposite a tip end face of said center electrode across a first discharge gap; and a second ground electrode having one end bonded to said metal shell, and another end face disposed opposite a side peripheral face of said insulator to form a second discharge gap between a side peripheral face of said center electrode and said another end face of said second ground electrode, wherein an area S (unit: mm 2 ) of an opposing face of said noble metal tip disposed opposite said tip end face of said center electrode, a distance t (unit: mm) in an axial direction between said opposing face of said noble metal tip and said inner side face of said first ground electrode body on a side of said center electrode, a size M (unit: mm) of said first discharge gap, a minimum distance F (unit: mm) in a radial direction between said side peripheral face of said center electrode and said another end face of said second ground electrode, a minimum distance A (unit: mm) in a radial direction between said another end face of said second ground electrode and said side peripheral face of said insulator, a minimum distance H (unit: mm) in an axial direction between a tip end face of said insulator and a middle point of said first discharge gap, and a minimum distance L (unit: mm) in an axial direction between said tip end face of said insulator and a tip end side edge of said another end face of said second ground electrode satisfy the following relationships when said tip end side edge of said another end face of said second ground electrode protrudes from said tip end face: 0.12≦S≦1.15, 0.3≦t≦1.5, A+ 0.7( F−A )≦1.8 M , and −0.3≦L/H.