Patent ID: 8278809

Claim:
A spark plug comprising: a center electrode; a generally cylindrical insulator surrounding the center electrode; and a ground electrode disposed such that one end of the ground electrode faces the center electrode via a spark discharge gap, characterized in that the insulator is formed of an alumina-based sintered material containing an Si component, a Group 2 element component, said Group included in the periodic table defined by Recommendations 1990 presented by the International Union of Pure and Applied Chemistry, or IUPAC (Group 2 may be referred to as Group 2A as previously defined by IUPAC, or as defined by the Chemical Abstracts Service (CAS)), and a rare earth element (RE) component; that the alumina-based sintered material has an RE-β-alumina crystal phase containing at least the RE component, and that the mean crystal grain size D A (RE) of the RE-β-alumina crystal phase and that of alumina D A (Al) satisfy the following relationship (1): 0.2 ≦D A ( RE ) D A (Al)≦3.0.