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. 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. 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.