An object of the present invention is to provide a spark plug having an insulator capable of maintaining a withstand voltage resistance in a high-temperature environment. A spark plug includes an insulator, a center electrode, a metallic shell, and a ground electrode. The insulator contains not less than 90.0 mass % and not greater than 98.1 mass % of Al2O3 with respect to a total mass of elements, as reduced to oxides, contained in the insulator, and has a porosity not greater than 5%. Assuming that a porosity of the insulator after a heat treatment is B, a difference (B-A) of the porosities before and after the heat treatment is not greater than 3.5%. In the heat treatment, the insulator is placed in a furnace, and a temperature in the furnace is increased from a room temperature to 1400 DEG C. at a temperature increase rate of 7 DEG C./min and is held at 1400 DEG C. for 30 minutes, and thereafter, the temperature is decreased from 1400 DEG C. to 400 DEG C. at a temperature decrease rate of 10 DEG C. or less per 5 minutes and is decreased to the room temperature.