A spark plug, comprising: an insulator having an axial bore extending in a direction of an axis; a center electrode provided at a front side of the axial bore; a metallic shell provided on an outer periphery of the insulator; and a ground electrode fixed to a front end of the metallic shell, wherein the insulator contains not less than 90.0 mass % and not greater than 98.1 mass % of Al 2 O 3 with respect to a total mass of elements, as reduced to oxides, contained in the insulator, and has a porosity A not greater than 5%, and a porosity of the insulator is such that, when a heat treatment is performed, the porosity of the insulator after the heat treatment is B, and a difference (B-A) of the porosities before and after the heat treatment is not greater than 3.5%, wherein the heat treatment comprises: placing the insulator in a furnace, increasing a temperature in the furnace from a room temperature to 1400°C at a temperature increase rate of 7°C/min, holding the temperature in the furnace at 1400°C for 30 minutes, and thereafter, decreasing the temperature in the furnace from 1400°C to 400°C at a temperature decrease rate of 10°C or less per 5 minutes and then decreasing to the room temperature; wherein in the insulator, a ratio of a total mass of an Na component and a K component as reduced to oxides to the total mass of the elements, as reduced to oxides, contained in the insulator is not greater than 200 ppm.. an insulator having an axial bore extending in a direction of an axis; a center electrode provided at a front side of the axial bore; a metallic shell provided on an outer periphery of the insulator; and a ground electrode fixed to a front end of the metallic shell, wherein the insulator contains not less than 90.0 mass % and not greater than 98.1 mass % of Al 2 O 3 with respect to a total mass of elements, as reduced to oxides, contained in the insulator, and has a porosity A not greater than 5%, and a porosity of the insulator is such that, when a heat treatment is performed, the porosity of the insulator after the heat treatment is B, and a difference (B-A) of the porosities before and after the heat treatment is not greater than 3.5%, wherein the heat treatment comprises: placing the insulator in a furnace, increasing a temperature in the furnace from a room temperature to 1400°C at a temperature increase rate of 7°C/min, holding the temperature in the furnace at 1400°C for 30 minutes, and thereafter, decreasing the temperature in the furnace from 1400°C to 400°C at a temperature decrease rate of 10°C or less per 5 minutes and then decreasing to the room temperature; wherein in the insulator, a ratio of a total mass of an Na component and a K component as reduced to oxides to the total mass of the elements, as reduced to oxides, contained in the insulator is not greater than 200 ppm.. placing the insulator in a furnace, increasing a temperature in the furnace from a room temperature to 1400°C at a temperature increase rate of 7°C/min, holding the temperature in the furnace at 1400°C for 30 minutes, and thereafter, decreasing the temperature in the furnace from 1400°C to 400°C at a temperature decrease rate of 10°C or less per 5 minutes and then decreasing to the room temperature; wherein in the insulator, a ratio of a total mass of an Na component and a K component as reduced to oxides to the total mass of the elements, as reduced to oxides, contained in the insulator is not greater than 200 ppm..