Abstract: A spark plug includes an insulator, a metal shell surrounding the insulator, a center electrode disposed in the insulator, with a front end thereof exposed outside from the insulator, a ground electrode having a fixed end portion fixed to the metal shell and a free end portion located at a predetermined gap apart from the center electrode, and a coating part formed of noble metal or noble metal alloy so as to cover at least a region of an inner surface of the ground electrode from a first intersection to a second intersection, where the first intersection is an intersection at which an imaginary line extending from an outer circumference of the center electrode intersects the ground; and the second intersection is an intersection at which an imaginary plane extending through a midpoint of the predetermined gap in parallel with the front end intersects the ground electrode.

Abstract: An ignition plug having a ground electrode that includes a base material layer and an erosion-resistant layer having a thermal conductivity of 40 w/m·K or more. The erosion-resistant layer extends at least from the center-electrode-facing portion to a location closer to the fixed end than a front end of the center electrode and 0.2 mm?thickness t1 of the erosion-resistant layer?thickness T of the ground electrode 30?0.6 mm is satisfied.

Abstract: A spark plug has a center electrode containing a shaft portion, a tip portion joined to a forward end portion of the shaft portion, and a joint portion joining the shaft portion and the tip portion together. In the spark plug, the distance between the surface of the center electrode and an inner edge of the insulator is 0.3 mm or greater. The inner edge is a forward-end-side edge of an inner circumferential surface of a small diameter portion of the insulator, the small diameter portion being a part of a portion of the insulator for accommodating the tip portion and having the smallest inner diameter.