Patent ID: 8390183

Claim:
A spark plug comprising: a center electrode; a substantially cylindrical insulator provided on a periphery of the center electrode; and a grounding electrode arranged such that one end thereof faces the center electrode through a spark discharge gap, wherein the insulator comprises a dense alumina-based sintered body having an average crystal particle diameter D A (Al) of 1.50 μm or more, and wherein the alumina-based sintered body contains Si component, a Group 2 element (2A) component containing Mg and Ba as essential components and further containing at least other one element excluding Mg and Ba in Group 2 elements in a periodic table based on a recommendation of IUPAC 1990, and a rare earth element (RE) component, such that a ratio of a content S (mass % in terms of oxide) of the Si component to a total content (S+A) of the content S and a content A (mass % in terms of oxide) of the Group 2 element (2A) component is 0.60 or more.