A spark plug 1 includes an insulator 3, a center electrode 4, a metallic shell 7, and a ground electrode 8. 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 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°C at a temperature increase rate of 7°C/min and is held at 1400°C for 30 minutes, and thereafter, the temperature is decreased from 1400°C to 400°C at a temperature decrease rate of 10°C or less per 5 minutes and is decreased to the room temperature.