Patent ID: 7250711

Claim:
A spark plug for an internal combustion engine comprising: a metal shell having a base end and a top end opposed to the base end in a lengthwise direction of the spark plug; a hollow porcelain insulator having a length which includes a body and an insulator nose, the body being retained within said metal shell, the insulator nose projecting from the top end of said metal shell; a center electrode retained within said porcelain insulator to have a top end protruding from the insulator nose; a main ground electrode which defines a main spark gap between itself and said center electrode; and an auxiliary ground electrode having a base end surface, a top end surface opposed to the base end surface, and an inner side surface extending between the base end surface and the top end surface, the base end surface engaging and directly being joined to said metal shell to orient the inner side surface to said center electrode, wherein the inner side surface of said auxiliary ground electrode defines an auxiliary spark gap between itself and the insulator nose of said porcelain insulator so as to occupy a minimum distance between said porcelain insulator and said auxiliary ground electrode, wherein the insulator nose of said porcelain insulator has a wall thickness T meeting a relation of 0.3 mm≦T≦0.7 mm, and wherein the inner side surface of said auxiliary ground electrode has an area which defines the auxiliary spark gap between itself and an outer peripheral side surface of the insulator nose of said porcelain insulator, the area having a length E in an axial direction of said porcelain insulator which meets a relation of E≧0.5 mm.