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° 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.

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