Patent ID: 8222803

Claim:
A spark plug, comprising: a shaft-shaped center electrode; a ceramic insulator holding an outer circumference of the center electrode; a metal shell holding an outer circumference of the ceramic insulator; and a ground electrode joined to the metal shall so as to define a spark gap between the center electrode and the ground electrode, the ground electrode having an opposing surface facing a front end of the center electrode, a back surface located opposite from the front end of the center electrode, a protruding portion formed on the opposing surface by extrusion pressing and protruding from the opposing surface toward the front end of the center electrode and a press recessed portion made in the back surface due to the formation of the protruding portion by extrusion pressing and recessed from the back surface toward the front end of the center electrode, wherein a protrusion amount A of the protruding portion from the opposing surface satisfies a relationship of 0.4 mm≦A≦1.0 mm; wherein the press recessed portion extends to a front end of the ground electrode and has a bottom surface region substantially in parallel with the back surface, a lateral surface region extending from the bottom surface region in a direction facing away from the center electrode and along which the press recessed portion is recessed from the back surface toward the opposing surface; and wherein there is a hollow recessed space defined and surrounded by the bottom surface region, the lateral surface region and a corner region extending from the bottom surface region to the lateral surface region.