Patent ID: 8564183

Claim:
A spark plug, comprising: a center electrode extending in an axial direction of the spark plug; a substantially cylindrical insulator disposed around the center electrode and having a small-leg portion at a front end side thereof; and a substantially cylindrical metal shell disposed around the insulator, wherein the spark plug satisfies the following conditions (1) to (5): condition (1): the insulator has a wall thickness T of 0.3 to 1.1 mm at an imaginary plane including a front end face of the metal shell; condition (2): a region of the insulator extending from a front end of the insulator to at least 2 mm rear from the imaginary plane is formed of an alumina-based sintered body containing a Si component, a rare earth element component and components of at least two kinds of Group 2 elements of the periodic table according to IUPAC recommendations 1990 and being substantially free from a B component; condition (3): the mass ratio R RE of a mass of the rare earth element component (in terms of oxide) to a total mass of the Si component, the components of the Group 2 elements and the rare earth element component (in terms of oxides) in the alumina-based sintered body is 0.15 to 0.45; condition (4): the mass ratio R 2 of a total mass of the components of the Group 2 elements (in terms of oxides) to a mass of the Si component (in terms of oxide) in the alumina-based sintered body is 0.25 or greater; and condition (5): the alumina-based sintered body includes, in a grain boundary phase thereof, an aluminate crystal containing the rare earth element component.